There are clearly good sides. The UK is still a big and powerfull country, nobody wants Irish or Scottish political instability, and it's a great morale booster to have a leaver come back.
But the UK had plenty of dark sides. They always wanted an exception for everything, with the pound not being an euro the famous example. The UK firmly pushed the EU in the 'economic union' direction, so now the advocates of deeper political unification get a chance. There was the only-half-joke from Yes Minister about how the UK only joined because they couldn't break the EU from the outside.
I presume the UK has not really demonstrated what really happens outside the EU. First Covid and now Ukraine are 2 major excuses for bad news, so the pro-leavers can still claim they are right. The EU probably wants to see some more pain in the UK first, to convince other doubters. But not too much.
So while the UK probably could get back in, it might be on much less favorable terms than they had in the past and take a while.