That's just my personal take, but I feel it was their constant desire to refer to DS1 lore that really had DS3 fail to stand on its own. DS3 was at its best when it introduced new interesting personas like Ludleth, instead of referencing yet another character from DS1.
The problem with DS3's handling of the original game's lore is that people at one point expected seeing 'answers'. Answers to what? They probably don't even know themselves.
Thanks to the absence of that kind of pandering in DS2, the game while weak in the story and character departments overall, still managed to create some level of intrigue and lead to a handful of interesting theories.
And theories is what this series lore lives off. Not off getting answers to plot threads and lore bits, but speculating about them. And yet they keep referencing and bringing up these old events, old characters that people have known for years, and build up these misguided expectations in them that everything from the first game and this one now needs an answer, although it never did. Because that never was the point.
Ludleth was an interesting character. We don't need to fight the beast that he did. We don't have to fight the dragon that was opposed to the Millwood knights. Some things happened and lie in the past. Not everything has to be connected to us in some way or another. That is what lorebuilding is. That is what makes a world feel rich.
It's interesting that, for a world founded on the downfall of dragons, there are a fuckton of dragons we both see and are referenced throughout the series. I don't think I've faced as many dragons in any other series as I did in Souls.
I don't play Monster Hunter, though, I'm sure there are a ton there.
It's not as much about needing to fight the dragon the Millwood Knights fought as it is just the absurdity of having two entirely different dragons (after their supposed extinction, seeing the Everlasting Dragon in Ash Lake was supposed to be a twist, not something that happens all the time) that are directly related to the abyss but not related to one another in any way, and in two DLCs that are supposed to be direct sequels to one another.
They had an opportunity for a payoff from the first DLC there while keeping their desire of a dragon boss, instead they continued to cheapen the lore with more dragons.
And I personally disagree with the idea that the most interesting part is just speculating on things with no answer. On the contrary, part of what makes Bloodborne interesting is that the Lovecraftian nature of the story makes it so it's actually pointless to try to understand the cosmic beings, they're beyond our understanding by design, and so it's a waste of time to keep speculating on their intentions, might as well focus on the concrete stuff.
I've always loved Souls storytelling, but because I loved figuring things out by myself. The fun part of discussing is pointing out things others may have missed, and learning myself, not reading random crazy theories on Velka or whatever. I want evidence, not other people's headcanons, I can make up my own for these things.
I've always seen the Souls method of narrative as if we're some sort of historian investigating a place and trying to figure out what happened from whatever evidence we can find. The vagueness just adds to that, we'll never have the full details because most of the important things already happened by the time we arive, we're just analyzing the present to uncover the past.