I'd just squish every expansion. Every old expansion gets added to the 10-50 bracket and newest expansion is 50-60.
In the end, leveling isn't progression anymore - it's just a way to tell the new story to players and introduce them to the new system an expansion brings. The real game starts at level cap.
So it's just more consistent to have all scaled back to current system. In the end level cap is just a number, and having it higher only means you have to fill up those level with meaningful stuff otherwise they'd feel just empty.