For me its quite clear what went wrong with PoE 2, it is direct sequel. Many people havent completed first part so starting direct sequel feel quite uncomfortable. Situation would totally different with another hero and story. Setting and lore is so great in PoE soni really hope they will make more PoE games. We just need a another hero.
Of course there were some quite derious flaws. Difficulty was balanced very poorly, games being much too easy. And on hardest difficulty levels game just start to feel like work, not challenging, just boring work. Another thing, when you playing crpg and reach level cap halfway game something is very wrong. These problem are in all newer Obsidianin games, PoEs, Tyranny and Outer Worlds. I still love these games but that is what separates from being true classics. And i believe these things annoy other hardcoreplayers too, and hardcoreplayers are core audience for these kind of games. If core audience appriciates then bigger audiences may follow after good word spreads around.
My tips for Obsidian in nutshell:
1. Dont do direct sequels, but use great settings you are already created.
2. Dont try to balance your games with all for everyone mentality, remember your core audience, and balance your games strictly for them.