זה היה מספיק
is very funny—I will propose it as an alternative to Dayennu at our next Passover meal: it will definitely crack people up.
Modern colloquial Hebrew has many fine qualities, but poetic metre isn’t one of them.
So that’s one reason why we don’t use that phrase here — but the second reason is that the true meaning of Dayennu is not “That would’ve been enough”, but rather: “We would have been content with that.”
Thanks for the chuckle, though…
ADDENDUM, March 31, 2018: We tried it at last night’s seder, and it was indeed hilarious…