Yes, as already pointed out, the rule doesn't work for all cases.
No rule will.
And the problem, which is .... you, would remain. And pretty soon, you'll be peddling some other nonsense.
So us "bad DMs" will just have to shamefully not have terrible players at the table.
Real pity, that. I'm already sad thinking of all the pointless arguments I won't be having. And all the playing I will be doing instead.
What about it wouldn't work?
Yes, changing the spell description.....changes the spell. In this case, although
hex could be
cast on a CR0 creature, it could not be moved
from a CR0 creature.
Yes, this is a change, but if the spell had always had this description, no-one would have batted an eyelid.
This solution (assuming that you think a 'solution' is required; I don't because I don't see a problem) works independently of the DM's mood; as it damn well should!
If your 'solution' to the non-existent problem of casting the spell exactly as written is "the laws of the universe might stop working, just because I don't like how you're taking advantage of them", then that is such a poor decision that you shouldn't have DMing responsibilities.
Player: I
disintegrate the floor so that I and all the mooks fall 100 feet, but I cast
feather fall on myself before I hit the ground.
DM: Gravity fails to act on the mooks, because that is an exploit! A clear 'bag of rats' situation, shenanigans! You are exploiting how gravity works in order to damage those mooks!
Is that DM response admirable?
Now try this:-
Player: I cast
hex 'on a creature that I can see within range'. I choose....that puppy.
DM:
Hex fails to act on the puppy, because that's an exploit! A clear 'bag of rats' situation, shenanigans! You are exploiting how that spell works in order to
cast that spell!
Other Player: I cast
hex on the puppy's dad!
DM: No problem! That's exactly how the spell works!
Player: WTF?