I feel like you could write that about most PvP game genres...? In terms of "they're well-tread and most fail."
Fighting games are the only genre with a regular emergence of new titles that see lower net success, but also lower longevity, while still being a thriving scene, and there's clearly something specific about that demographic that can pay off in that sense (with most that strive for eternal longevity failing). This clearly does not pan out in any other PvP genre -- arguably some studios over-invest, but there are more PvP indie and AA games now than ever before (like, seriously, sooooo many), and most of them fail outright so hard that it's impossible to argue that large budgets and marketing aren't a factor in PvP games succeeding (i.e. getting the player counts needed to even work).
The shooter genre has at least added some items to their list -- now it's a CoD, a battle royale, an extraction shooter, a Counterstrike, or an Overwatch. It used to only be 3 of those things. Valve could try to make 1 more, and if they did, there's a 90% chance it would fail and no one can control that outcome -- it's not quality, it's not passion, it's not innovation, nothing controls what leads to cultural embrace of a gaming format, it's effectively random (within obvious constraints of marketability and fun). They're basically trying that here -- this isn't a new effort, but maybe they can get lucky and make the breakthrough MOBA shooter. I'm sure everyone working on it is thrilled to try, but I agree with you, there's a huge chance it'll fail -- I just don't think that's much different than the huge chance for any PvP game to fail.
Because actually Valve already released two entirely non-standard and now failed PvP games. Their autochess game was was part of a new genre when it came out and it failed, Riot ate their lunch and now has one of the most successful casual PvP games ever made. There was no lack of polish or passion or unique efforts there. Even more importantly, their card game was ridiculously innovative and it also failed -- maybe in part due to its financial model, but who's to say how different it may have been otherwise. (Riot's card game is still struggling despite having the best card gain model on the market and being one of the most beautiful and effortful and passionate endeavors into a DTCG possible.)
So... I sort of get your sentiment, given Valve has infinite money and could very easily be putting out more (on cheaper budgets, on more creative fronts), but it's also not surprising to me at all that they'd try a safe bet that they understand and are passionate about, nor that a lot of people working there like working on PvP games specifically so that got enough traction to get the buy-in of their bizarre "everyone does what they want" creative structure.
