I never even played 4e, but from the sound of it, it at the very least had some good design in it.
It definitely did.
It wasn't perfect or even close design-wise. There were major mistakes, among them:
1) It was rushed out (so was 5E though) which left the overall mechanics flatter than the designers really wanted (they improved this later but it was a lot later).
2) They offered too many abilities which were Reactions (of which you could make several in a round), Interrupts (to someone else's turn/action), Immediate actions (usable at at any moment), especially from level 10 and up. These served to massively bog down the game.
3) Someone senior at WotC but seemingly not one of the main designers literally
secretly increased the HP of everything in the MM shortly before it went to print. This is mysterious as hell and I'm sure WotC people know who it was but they're not saying. This made them work way, way worse. Again they fixed it, but not until later.
4) They let Feats get completely out of control. Again. They should have learned from 3.XE on this, but they totally didn't. It might even have been worse than 3E here, in real terms.
Most other mechanical/design things were a matter of taste/perspective, like not for everyone (what is?) but not "bad" design, and in some cases extremely good design.
worse than 5e? I don’t think marketing helps all that much if your core audience rejects it
Hugely so. Absolutely night and day. 5E's marketing was clean and straightforward and avoided any major mistakes.
The marketing CAUSED the core audience to reject it even. The main marketing campaign literally implied that you were stupid and bad for liking older versions of D&D. Like I said, it was like a Simpsons bit - it was even narrated by someone who sounded like generic "Snooty European". I was amazed that didn't go even worse than it did based on that. At the same time WotC's then CEO was saying insane stuff like "I want D&D to be WoW!" (by which he meant something people play online with a subscription, in this case via a 3D VTT), which like, no-one thought was a good idea in 2008.
not sure how much that actually matters in reality.
I obviously don't agree and your rationale is "WotC
had to back down when it tried to do it again and they were fine!" and you're ignoring the bolded bit. I think 5-10% audience loss would be true in the short term, but five years from now? We'd have seen bad consequences. We may still.
same with 2e to 3e, 4e to 5e. They did it at a time when sales were low, so not releasing a new edition would not have done them any favors either
And none of the others had an alternative waiting in the wings. There was no Pathfinder equivalent for 4E because of the GSL.
which only matters if they do not like your new game better, and in that case you have a problem already anyway
My point is that most groups don't have a strong preference for new edition over old edition and don't care about the details of the rules very much. But a snooty European (like me) telling them they're dumb losers for liking 2E/3E, the CEO saying "D&D will become WoW!" and so on sets you off on a very bad footing, that no other edition remotely faced.
More to the point you're engaging in revisionist history of the bad kind, because you're acting like people knew 4E's rules before the bad marketing happened, but that's completely false. The bad marketing, the GSL, etc. that was all known for months before the details of 4E's rules were. People were really mad with 4E already when the rules hit so they were basically getting out of limo into a mob full of villages with pitchforks and torches.