Heroes was not too late. With matching up Blizzard characters from all franchises up against each other it should have been a huge success.
It only wasn't one by Blizzard standards TBH. While a nice timeline and exact number of players over the years with a clear peak is hard to find, it seem it did reach 6,5 million (unique?) players within a month at one point, which by the same metric was over half of what DOTA 2 had around the same time, while LoL dominated with some obscene number of 100 million of I guess mostly gooks:
https://twinfinite.net/gallery/most-played-games-ranked-monthly-players/5/
The problem is of course that it didn't last on a BW/WoW like timescale, although the above was still what, about 3 years after HotS was released? Normally this would be a huge success as far as online multiplayer games go, I mean the above list is a top 10 most popular and HotS was 8th back then. Games like BW, CS etc. that dominate for over 10 years are outliers, while the rest comes and goes, so by the numbers shown above it was huge success, just not a legendary multiplayer timeless classic kind of success. The question here is though, how much money Blizzard burned along the way to try to get good numbers? They did decide to kill off the game eventually for some financial reason, so maybe the game did flop from a RoI point of view.
I'm inclined to believe that for all it's flaws if HotS was released in the tail end of the 00s before or around LoL it might have taken its place as the "more accessible/casual" alternative to DOTA. Although the big IF here is the individual expression and connected to it lack of items that you mentioned. Contrary to what Blizzard thought or banked on with HotS design centered around team play, the mass appeal of the genre might be more of a snowballing powerfantasy, where people/primadonnas get to stroke their egos by getting their hero fed to the point where they can claim they won the game for the 4 useless noobs on their team, showing off how "good at the game" they are.
Blizzard probably actively tried to tackle that with HotS design, but that might have been kind of like attempting to make vodka that wouldn't have alcohol. Every other "bad" thing about the game though, well it is not hard for me to imagine how people would get cargo cultish about slow heroes or other bad according to you design choices, creating justification why it's good for the "competitive/spectator" side of it once the game got popular. People watch real life ballsports, FIFA (or how is EA calling it now) is a popular e-sport, and there's probably a bunch of other examples that people will watch or follow of absolutely unspectacular games or boring shit TBH. Good gameplay is not a strict requirement, it just has to be good enough for the masses or flip the right psychological switches.