Well, the problem is that you can have hand-routed designs and other optimizations that only apply at certain feature sizes. Most analogue design work won't carry directly over in a die shrink.
Very true.
But if process migration is a high priority from day one, it
is possible to make easily-scalable layout. As in, migrating the layout takes ~10% of the time required to create the first version. But you have to design the original with this capability in mind, and most people don't do that.