Have you thought about the consequences if everyone bought furniture like that? There's a reason why solid wood furniture is more expensive, it's a LOT more resource intensive and wasteful during manufacturing. Imagine if everyone started buying all solid wood furniture tomorrow? How many more trees would be cut down? How much wood fiber left over from making other products would be wasted because people wouldn't want it for furniture anymore?
"Ikea was a terrible thing to happen to furniture - disposable single-use furniture - madness."
I haven't seen a single person back this up with any facts, i.e. - what's the resource use per year of an average IKEA furniture and the alternative?
I'm willing to bet you and others are just making a giant assumption, based on your own personal annoyance that the furniture isn't as solid as you'd like.
I'm not going to make a strong counter-claim. Yes, IKEAs way could be worse, but I've yet to see proof of that, and you can make a pretty strong argument that IKEAs way has a potential to be more sustainable. Especially if they manage to recycle the wood fibers efficiently.
"Many people say you essentially can't move Ikea furniture between houses because you can't dismantle it and if you tried to it'd simply fall apart." - In my experience that's simply wrong. I just disassembled/reassembled a relatively new IKEA wardrobe during renovation. I did not handle it gently. It was no problem. Yeah, you can get some damage, but nothing a bit of glue, an extra screw and/or plaster+paint can't fix easily. You can get catastrophic damage if you're not careful, but I'm willing to bet that the probability of that is low enough that the global impact on average lifetime is low.