The design change still came from the new book, not errata. It only got added to errata after the book was made. "Promise" kept.
No, I have no problem with players who don't want to adopt these changes. Those players still have all their old books and can ignore every new book they want for all of me.
My problem lies with the belief that the designers aren't allowed to continue iterating on the game in any published form until 6e. That is, yes, stagnant.
"What they've said previously" in that apocryphal vow you've yet to produce?
So if I say something like "leveling progression and quest design" that's too vague, but if I get more specific about what they changed it would be a waste of time anyway because, by your own admission you've never played any MMO before so you'd have no idea what I'm talking about anyway. So how exactly does that help?