> ...you still have to do an ALTER TABLE if you change the underlying type of a column.

I don't see why this is a problem? How often do you update really big (hundreds of millions of rows) columns' data types in practice? Most of the time you'd change type early in development when you don't have that much data in the table yet which makes it pretty much instantaneous. If you really need to update the data type for a supersized table you could always make a new column instead and migrate lazily to that in the background. But all of this happens so rarely that in my view it's a silly excuse to dump all the great benefits you get from having an enforced database schema.