Having to wait until next season to promote isn't lost on me. I tried to beat an established IV in my first season up, and got my ass handed to me in the playoffs. I can relate to your example. That team went undefeated and promoted, and I had to play another season in IV. It made me better. I didn't deserve to promote because I wasn't the best team in MY league.
If you want to analyze all the teams who got stuck behind juggernauts, and had claw their way out of tough competition, go for it. It will happen every season. But don't tell me they deserve to get cleanup promotion based on their league strength. That's just undervaluing to teams who planned out their championship runs carefully albiet in weaker leagues. Their championships aren't a sham.
Besides, the men are separated from the boys through promotion, when the teams that weren't ready fall right down. The ones that got knocked around a bit in tough leagues usually stay up.