Helldritch
Hero
Unlucky with the breath recharge, but not with the criticals.So you added spells to a monster and used lower level spells to kill half the party...
..and that proves my point that the spell slot paradigm of 5e is not made for many uses and if a monster gets down to lower level magic that they will kill the party wrong?
Because it sounds like if you weren't unlucky then the players would be making new PCs.

And again you reached wrong conclusions. It is not about TPK a group, it is about versatility. And spell casting dragons are already in the MM as an option. It does not even modify the CR.
I only played the dragon optimally and the players were very Unlucky leading to a very long battle but exciting as possible as thee scales could have tip in favor of one or the other. In the end the dragon left to save its skin. Had he stayed, it might have TPK the group, or have been stroke by the ranger's arrows and die. When it is 50/50 chance, you simply leave to fight another day.