Either the game is about the regular season or the playoffs. Your argument would also apply to those teams that win the playoffs at the expense of teams that were better than them during the regular season, so I find it irrelevant. The game is all about the post season. The regular season is just a prelude. As it is in all real sports that have a playoff system.
I agree 100% with jokeru. Playoff performances should count for more. Only use regular season performance to decide which runner up promotes.
Though of course personally I'd rather see us go back to just the division winners promoting.