[5e] When to swap out polymorph?

Polymorph is a fun and powerful spell, generally used more for its party-buffing capabilities than its enemy-disabling ones.

But it has a pretty hard cap: a level 8 player can be turned into a T-Rex, and then that's it, that's the strongest you can get them. At level 11, you still just have a T-Rex.

So when does this stop being a productive use of your spell slots and concentration? When are you better off swapping it out, on a spontaneous caster?

Secondary question: does the answer to this change if you can Twin Polymorph?