Another thing to keep in mind is that 3e is functionally a pro-player system, and was built that way in response to feedback gathered by WotC. It restricts the DM to being mostly a narrator and rule-Googler, and philosophically suggests the DM has no real power to affect the outcome of the game aside from making individual NPC decisions. Meanwhile 5e is very much a pro-DM system, with many rules and followup tweets and errata encouraging the DM to make sweeping creative decisions that affect the overall system. This is a fairly deep distinction that, I think, gets obscured by complaints about 5e being too simple.
People who like 3e often do so because it's a game that empowers the player over the DM through system mastery. It's the "peoples D&D" in many respects. 5e comes along and tells us system mastery isn't as important as what the DM wants to have happen. That has to stick in a lot of craws.