An example - a big SP game costs 200 million
Only 3 games(4, when adjusted for inflation) ever made have had a budget over $200m — with the majority of that budget typically going towards marketing.
Of these four:
Within 2 months, the #1 game had made over $1 billion in sales.
Within 24 hours the #2 game made over $800 million in sales, going on to gross $6 billion in 5 years.
#3 was a flop. FYI, it's the only online-only game out of the four.
#4 recouped its development cost in the first 24 hours, going on to sell over 6 million copies.
These games are the massive outliers.
Something you didn't mention is that it costs far more to produce an online game than a singleplayer game does. It requires additional server development, additional infrastructure, and is also typically expected to have far more content while being continuously supported after release.
A multiplayer garbage like Hearthstone or Overwatch costs a nickel and makes half a billion. Every-fucking-year.
If it was so profitable then companies would be doing it. Guess what? They did, and it wasn't. Basically every major gamedev tried to get some of that money and they all had one thing in common: They failed. Even the ones already with a wildly successful online-only game failed.
Do you remember Heroes of the Storm by chance? The shoe was on the other foot when Valve tripped with their hearthstone-like game Artifact. Epic may be known for Fortnite now, but Paragon tanked. Digital Extremes(known for their wildly successful online-only looter shooter Warfame) made The Amazing Eternals which never made it out of beta because it never had a big enough community, and there was Realm Royale from the devs of Paladins.
Hell, basically every big company tried to get some of that. And again, most of them failed. Gearsoft's Battleborn, CDProjekt's multiplayer Gwent game, CDProjekt's Witcher moba(yes, this existed), Turtle Rock/2K's Evolve, Square Enix's Lord of Vermilion Arena moba, Warhammer Online: Wrath of Heroes — because apparently Mythic didn't learn their lesson from Warhammer Online already. Cliffy B failed twice and left the gaming industry: LawBreakers and Radical Heights.
Oh Cliffy B.
Oh hey, and since we're discussing Bethesda maybe you guys have heard of Quake Champions?
To be successful as an online-only game you have to be capable of beating the entrenched competition, typically through being the first or leveraging an already massive fanbase(which has often failed as I've noted.) If you think there's easy cash to grab then you simply aren't paying attention.
You know what big businesses really hate?
Taking risks.
If anything, you should be more worried that "safe" games like Outerworlds become the norm. Shallow games designed to sell as many copies as possible made on medium budgets and offer nothing of note. Games that are hollow shells of much better titles with just the barebones necessary to make people really feel like they're sort of playing a game they like. Games that produce steady, guaranteed returns for investors.