Agreed, this kind of reaction is more, I guess, understandable or warranted if the software project was being developed and hosted by large corporations in closed source; which thankfully Lemmy is neither.
It's thanks to these volunteers that Reddit Admins/CEO can't put leashes onto us and force us to dance around for them. It's thanks to these volunteers that OP can voice their opinion, albeit in a demanding and spoiled manner. Without these volunteers, Lemmy would've been in a worse condition than it is now.
So OP ought to be more grateful to these volunteers.
Don't lump all of us into your demands. Some of us like the new UI. And besides, stagnation is never good, we'll miss out on bug fixes and new features if we went by your demand.
What should instead be done in my opinion is to allow users to choose between UI styles if the default UI style isn't up to their taste.