AND DESPITE ALL THAT AND YET, I've never seem this games explode like some big milestones in gaming history, like MOBAs, Minecraft or Battle Royale games. If everyone and their mother in the gaming community craves for them (even with the difficulty barriers and such) I think this games should be way bigger than its fan base makes it out to be.
Games, franchises and entire genres can be popular and profitable even if they don't reach the absolute top floor of popularity.
The inconvenient truth with gaming is that a non insignificant portion of gamers are kids and young teenagers, it will always be like that and that is bound to heavily shape the market. There are some genres that are almost completely indigestible to those audiences, because they are willingly or unwillingly obtuse, not flashy or accommodating, and require a more proactive mental effort to be understood and a lot of patience for relatively little emotional reward.
Of course I'm not saying there a game either appeals "to kids" or "to adults", there are hundreds of sub-groups inside each with their own preferences, but whenever one of the two main groups is almost completely precluded to you, you are playing on a different league from the start. Put a Soulsborne game in the hands of kids or young teens, 95 out of 100 will get frustrated very quickly and their first reaction will be "this game is stupid and unfair" rather than "OK, I need to re-think my approach". I know I would have if I had tried to play Bloodborne when I was 12 or 13.
Every "milestone" as you call them is simply something that appeals to both kids and adults on a large scale, and rakes in the big numbers with both. Failing to do that doesn't mean a product is not popular or it doesn't sell. Assuming something has to be the "best" seller in the universe or you should not even bother making it is pure hyper-capitalism retardation that only leads to industry collapses in the long run.
Also Cleric Beast is optional. Father Gascione is first required boss:
Keep in mind Bloodborne was given with PS Plus at some point, and there were likely millions of people who had zero interest in it before, opened it once just to see what the fuss was about, NOPEd out of it after 15 minutes and never looked back. With that in mind, these are actually pretty impressive numbers for a supposed "fringe" genre.
If you look for example at the Trophies of Horizon Zero Dawn, which is supposed to belong to one of the big "mainstream" genres of our times, there is still a clear 25-30% of users who dropped it very very quickly. 34% of people completeld Horizon's main questline, and 25% of people defeated Mergo's Wet Nurse in Bloodborne (which happens very very close to the end, but due to the multiple endings with different Trophies - and the fact some people did more than one - it's very hard to gauge story completition from those). There are actually more people who got the Bloodborne's Platinum (6.4%) than Horizon's (5.9%), even though they are both very straightforward and only require you to play the game in full with no particular cryptic nonsense or grinding required.