Any assumption made in order to ship a product on time will eventually be found to have been incorrect and will cause 10x the cost that it would have taken to properly design the thing in the first place. The problem is that if you do that proper design you never survive to the stage where have that problem.
I think the solution to that is to continuously refactor, and to spell out very clearly what your assumptions are when you are writing the code (which is an excellent use for comments).
Continuous refactoring is much easier with well constrained data/type schemas. There are fewer edge cases to consider, which means any refactoring or data migration processes are simpler.
The trick is to make the schema represent what you need - right now - and no more. Which is the point of the “Make your invalid states unrepresentable” comment.
I think the solution to that is to continuously refactor, and to spell out very clearly what your assumptions are when you are writing the code (which is an excellent use for comments).