I actually really like the comparison between mainstream music and video games. Never saw someone doing that.
Because, what is important in this kind of comparison is, that the quality of a game or music has nothing to do with how popular or marketable it is.
Pop music is very easy and straight forward for the most part, easy time-signatures, no whacky meters or (poly)rhythms. But that doesn’t mean it is from “low quality”, quite the opposite. Mosts pop music is astoundingly well mixed, mastered and produced. (even though a lot of people that don’t like pop music that much would say its mixed very stale and boringly)
The same can be said about blizzard to keep that example. Most of their games are very approachable, hence why they are so successful in terms of marketability to a wide audience. But they quality for the most part is outstanding. Combat feel, music, cut scenes, polish, bugfixing are all on a very high level, even though a lot of these games have very simple mechancis and features.
Also blizzad probably has the single best CGI trailers of any video game company in my humble opinion. Even though I am not a big WoW or Diablo fan, their CGI stuff is top notch and impressed me all the time.
On the opposite side of things there are games that are not really “mainstream” viable, but yet are still high quality. Most of theses games I would name here as example are not really ARPG’s though.
So staying with the ARPG comparisons I think Last Epoch and Path of Exile are the best to directly compare it. Both have qualities in their own right, but they are not really mainstream viable the same way Diablo 4 is. I don’t know the exact amount of player numbers, but I think Diablo has or at least had multitudes of playernumbers higher than both LE and PoE combined. Diablo has a huge fanbase and combined with the mentioned approach of blizzard making very accessible games this makes the game very “successful” (in terms of player numbers).
But a game (or the devs of that game rather) can choose to stay within a certain “niche”, even though something else would be more “sucessful” (player count, monetarily etc.).
Overall ARPG’s are not the biggest market in video games. Even if you exclude mobile games other genres are multitudes bigger than ARPG’s.