The player retention problem is not exactly something that you can just easily fix and I have high doubts going F2P will solve anything and instead only make companies become shameless in their monetization. They did try F2P years ago with Tekken Revolution and KI, both of which didn't catch on with TR being shutdown completely and KI being dead. All F2P is going to do is give a company like Capcom that has had a history of shitty business practices to continue exploring those shitty business practices under a new guise.
As for player retention, the truth is this: the modern day gamer is a bitch. That's all there is to it. You can make one button specials, one button supers, F2P, tutorials that go into every painstaking detail, rollback netcode, rank based matchmaking, etc. and it won't matter because the modern day gamer fears failure and in a one-on-one fighting game which requires you to be the one to solve the problem your opponent presents it just doesn't work out for most out there these days. There's no team to fall back on or get carried by, there's only you. And when you are new to a fighting game and you're facing veterans, you're not only going to know it but you're going to feel it's insurmountable to ever get to that level.
Fighting games and RTS' in particular are brutal when it comes to the new player experience. It's why the best time to learn any fighting game is always going to be in the first two weeks of a fresh launch since you will at least have people either on a much more level playing field or have other new players to bounce off of.
But it's not something you can just enforce or make people do, they have to want to do it themselves and come to terms they're likely going to get fucked hard on the way to becoming proficient or decent enough to eventually present a realistic challenge to better players. Single player doesn't funnel them to online, F2P will at best result in a curiosity trip for an evening and then an uninstall the next day, making the inputs braindead easy just makes a good player no longer have to worry about execution and blow these new players up even quicker.
Reason why a game like Smash or this Multiversus thing are thriving is because of the huge brand recognition and that those party-brawler or whatever you want to call them games in particular are so easy to pick up and play it's feasible that a novice could steal rounds or entire sets off a more experienced player. Now imagine a novice in KOF XV or Tekken 7 trying to get victories over a veteran of the series, it's like they're playing two completely different games and crossed over into one another's world by accident.
It's even why battle royale games are so popular. The idea that "anyone can win, they just need to be at the right place at the right time" makes people feel they have a chance to win regardless. Fighting games don't have that because to win in them you need to actually learn and practice and most players out there don't find the idea of studying frame data and standing in training mode practicing counters or combos or set ups for hours to be worth the time investment for a rush they could easily get elsewhere with far less hassle.
If you really want player retention, make crossplay the standard going forward.
Killer Instinct didn't really try free-to-play. It was more like some pseudo free-to-play. More like a one character demo. It was also locked on a system for basically it's whole lifespan that Don Mattrick killed before it even launched; not only did he kill the system, but he killed the fighting game genre having a place on that system when he (or someone at Microsoft) basically told Capcom to fuck off with Street Fighter 5. When they lost Street Fighter, they lost that fighting game audience that would've probably nought other fighting games too. And when they lost their top spot from the 360 era they lost any reason for smaller Japanese fighting games to even think about the Xbox One as a platform.
Tekken Revolution isn't really the best example either because it was just a F2P variant of Tekken Tag Tournament 2 from the year before, and it was locked to the PS3. Being locked to the PS3 ain't exactly as bad as being locked to the Xbox One, but if you're an online game during that time the 360 is probably where you wanted to be. It was however downloaded 2 million times within its first two months.
Brand recognition doesn't matter half as much as people think. It helps. Sure, it helps, it's going to get more eyes on some thing than nothing at all. But it doesn't really fucking matter. The biggest, most profitable fighting game of all time is still Street Fighter 2 because of all the money it was racking in during the arcade days. Street Fighter as a franchise is still one of the most profitable video game series of all time because of those first three (I think it's the first three) versions of Street Fighter 2. Now did Street Fighter 2 have brand recognition when it came out? Fuck no. I know it's call Street Fighter 2, but nobody knew what the fuck Street Fighter was when Street Fighter 2 came out...which was probably for the best. Many of the most profitable games had no brand recognition before they got big. Fortnite didn't get big off the back of some brand people knew, it got big because it was free. Dungeon Fighter Online didn't didn't the most profitable game because of a known brand, it did so by being free. Even some known brand like Call of Duty didn't become the juggernaut it did until it revamped its whole brand with Modern Warfare four years and main games into the series.
The bit about KOF XV also being too hard in comparison to Smash is fucking stupid too, and here's why: Street Fighter 2. Like I said, the Street Fighter 2 series is the most profitable fighting game franchise. It's the most profitable by a wide margin. It became the most profitable by having such a low buy in cost, and by just being really fucking cool at the time. People might not exactly be jumping to spend fifty or sixty bucks on whatever new fighting game, but they sure as shit were willing to throw a quarter away to fuck around on an arcade machine for a bit. The free-to-play mode means your buy in is nothing. You just need to get people to want to download it to try the thing out. Now, would simple move executions like Smash, Buriki One, DNF Duel, and the upcoming Street Fighter 6 keep (or get) more players in? Sure. But if it's free you're going to get people regardless. And if your matchmaking isnt throwing people that are shit into fights with players that are just raping them all the time you're probably going to keeping them; although that could happen in the arcade days too, and it didn't stop enough people from playing to hurt anything.
Cross play is probably on the lower in of the totem pole of what's going to keep people playing. It'll help, because it means you're going to have a larger pool to pull players from, which means matchmaking will go faster. It's also maybe going to mean the player can jump between this or that thing to play the game. But I'd put good net code above cross play. Most people probably aren't saying, "I'm gonna get this for the netcode" it's just not something I think most people think of, but, and especially if your a F2P game, if the game doesn't feel good to play because it's too laggy or something, people are just going to fuck off to something else. Simple controls would probably help more too. I used to play tons of fighting game, and I've got friends who played fighting games, and friends that didn't really play fighting game, and something all the friends that didn't really play fighting games had in common is they had a harder time doing shit. It's probably no surprise that Capcom's flasher Marvel games, which did away with double motions for Hypers, and let you do Hyper moves by just pressing two buttons, were their better selling fighting games post-Street Fighter 2.
