The sim engine couldn't have been re-done without the Action Tracker, so maybe there could have been custom season lengths, but we'd still be stuck with the same broke *** sim engine from years past. I don't think anyone who plays Be A GM would want that.
Also to your last point, this doesn't just cover simming user games, it covers the entire landscape of your GM mode and the results that are churned out when it comes to player stats and team records. Even if you played all 82 of your games, this still covers your GM mode too because it's simulating what's going on with the CPU teams.
I'd much rather have this to be honest. Not to downplay the people who want custom season lengths, and it would have been nice to have both of these things, but IMO this was a more important thing to do this year.