Re: All 745s before any 720s, very likely. Re: All 755s before any 745s, I'm not sure about that. I suspect there may still be some 3-car units that haven't entered service by the time the first 745s are up and running.
Having travelled alongside them for many years and being one of them myself, there will be no pleasing them. People that send abusive tweets when their service is delayed by 4 minutes due to something entirely out of the TOC's control like a passenger passing out, will never be satisfied. Their opinions to some degree are irrelevant and especially so when we all know full well new stock is indeed on the way for them too. That they might have forgotten would simply be due to Abellio's not unjustified decision to remove the stickers about 'new trains' given how late they are being delivered.
The 357 option is also less than half the price per-mile, more punctual and was once upon a time less crowded (now the reverse after a lot of Southend area people jumped ship), the problem is, there is no parking at Upminster beyond about 7am :P If there were, that's the way I would commute.
I wouldn't describe the 321s as uncomfortable when it isn't hot outside, but will certainly be glad when they are replaced, put it that way.