If you want to be waterfall, that's fine. If you're forced into doing it by your business context, that's also fine. But, you shouldn't be under any illusions about what you're doing. It's a waterfall behavior that will drive waterfall effects.
Relative sizing is still an estimate.
> They're for re-arranging the priority of stories and deciding which ones to do or not.
Hard disagree - that's what priority is for. Story points are an _estimate_ for how much we can do in a period.
Not one which would attract any pressure.
If you don't think toxic managers and scrum masters are going to use that "commitment" to death-march the team if it looks like the sprint goal is going to be missed then you have a far more optimistic view of humanity than I do.
Scrum does that because it considers waterfall on a 2 week cycle to be training wheels for people who have been doing it on 6 month increments and because it considers that "closer" to agile.
Note that until relatively recently (2020, I think), the Scrum Guide referred not to "commitments" but to "forecasts". That was a much better framing, and I don't know why they changed it.