That didn’t prevent Norwich and Blades going up
This is certainly true, i.e. under Megson our first promotion was done on merit over money - but as a general rule, it's MUCH easier to go up when you also have financial clout. Look at Villa for instance, swinging money around like it's going out of fashion and it wasn't pretty at times, but eventually they went up. Same with Wolves, a few failed managers but eventually it clicked - whereas when they were poorer they struggled.
I don't think anyone's saying you're doomed to 20 years of non Premier-league football, but the issue is it gets harder with every year. Birmingham have been in the Championship close to 10 years now, so it can creep up on you, some clubs even more so.