That last sentence was hilarious. Gosh I just need change so **** badly you're right!
Anyway, theory fighter's worthless because I can think of a dozen situations to bring up where Rollout -isn't- worthless, and a dozen more situations where it's improved because you can shield cancel it, but then you'd counter why it's actually worthless there or there's something better to do, but then I'd counter how why it's not worthless and how that something better could be a bad decision for this this and this reason, and so on. We'd both feel like the other's completely misunderstanding us, and how clearly right we are.
Actual matches and their outcomes rely too much on figuring out your opponents habits and capitalizing on various mistakes that theory fighter just doesn't account for, and this sort of change works in that realm.
Here, I'll give a silly enough example. You charge rollout and release, your opponent starts charging an fsmash to hit you once you're close enough, but you just shielded! Now they're stuck charging and you can capitalize once they're forced to release. So people adapt, and learn to just smash once Puff's close enough. So instead of running into it you shield, not only shield but powershield! With such little shield stun you jump out and shfair or whatever. So there's ways to adapt to that? Well you should know that people aren't always perfectly familiar with matchups, and even if this attack has a perfect counter, you might notice your opponent's not really familiar with how to fight Puff and take advantage, thinking they'll not know enough and fall for one of your new tricks available with shield canceling.
It's like upthrow-rest in Melee. Sure, the spacie should di it and avoid it everytime, yet even really good spacies in really recent matches mess up and lose stocks that way.