Indeed, the old school assumption is that monsters exist for a different reason than player characters, and thus are designed differently. And specifically, the short expected lifespan of a monster is a large part of that difference.
Polymorphs are one of several situations (including monstrous PCs and cases where monsters are simply used for purposes of more substance than a one-minute battle) that break the game because of those assumptions. If you only ever use monsters for short battles, than they can be balanced on that basis and it will work. However, if that is not the case, it doesn't work. And since for many people it isn't, the game needs to evolve to standardize and balance all characters in the same way. While humans, cats, golems, angels, and dragons are very different creatures, they all need to be built and advanced in the same manner. In this regard, 3e was a step forward from 2e, and 3.5 was a step forward from 3.0 (and PF hasn't done anything and 4e goes off the rails and 5e is not entirely back on them). There's still work to be done in this area.