Polymorph Exploit.

What if you used the Polymorph to turn a villain into, say, a fish or another creature that couldn't breathe in air. If they suffocated from lack of air, would they die from Exhaustion? RAW, that would seem to be the case. 

If so, that's a huge exploit, because normally if you just attacked it, it would revert to its normal form.

Under pure RAW, this would technically work - the creature would suffocate, hit six level of exhaustion, and die.

Under Rules as Intended, I think it is reasonable that a DM rules that a polymorphed creature who would die of exhaustion reverts to their original form and thus is able to breathe and loses the exhaustion levels.  This is personally the option I would choose as I think allowing a 4th level spell to be such an easy kill would (a) be unbalanced, (b) not be fun, and (c) would strongly lend itself to main character syndrome problems and other players feeling like the Wizard can just solve every fight on their own.

I might go so far as to say any DM who allows such an exploit has failed every other person at their table.

Yes, probably, though there are a few things to keep in mind with tactics like this.

One is that a lot of high-level enemies have abilities like Legendary Resistance that makes it very difficult to successfully cast spells like Polymorph on them.

Another is that Polymorph ends immediately if the target loses all the temporary HP it gets from it, so if you choose a very weak form it may be trivial for the enemy to end the spell quickly just by damaging itself, or by its allies damaging it.

And, of course, Polymorph is a Concentration spell, so the enemy or its allies can end the spell early by damaging the caster or making them Incapacitated.

And also that the enemies can also turn the PCs into fish.

"If I allow that to work for you, it would also work for your enemies, and that would be no fun for anybody" is a very useful explanation for why you're shutting down a "clever exploit".