Then if the column was altered from NUMERIC with scale to be a plain NUMERIC, it would have to rewrite the table to enforce the row-wise scale to match the old column-wise scale. Where as now that alter doesn't need a re-write. I don't know if this would be an overall gain or not.
We've talked about cases like this in the past. It's mostly a SOP and I think it may already be on the TODO.
The main difficulty is that Postgres is very extensible. So to implement this you need to think bigger than NUMERIC. It should also be possible to alter a column from varchar(5) to varchar(10) for example (but not the other way around).
One way to do it would be to extend pg_type to have another column that specifies a function. That function would take the old and new typmod (which is what stores the 5 in varchar(5)) and tell the server whether it's a safe change to make without rechecking.
Another way might be to overload the cast functions, though they currently receive no information about the typmod. They might have the benefit of being able to handle things like varchar(5) -> text though.
But it has to be that general. Any data type should be able to specify whether an old and new typmod are compatible.