Indeed I don't think the new UI was caused by a desire to increase engagement. All the dark patterns and annoyances can trivially be implemented on the old UI, up to the slowness if needed (just add sleep(500) on every HTTP endpoint).

I believe the problem is that there's a significant frontend/JS team (along with managers, etc) that needed something to do so a rewrite was planned. It probably started with good intentions (and the initial MVP/prototype probably was lightning-fast), but now once all the bloat is they ended up with a pile of shit, but bonuses have been paid, people have been hired/promoted and can brag about this new "achievement" on their resumes, so turning back while saving face is impossible.