You don't want to discuss why you think this idea is necessary, but I think that's an important question to bring up. What are you trying to fix with this change? That's not changing the subject, that actually matters.
Whatever it is you are trying to change, it is unlikely that it is broken in the first place. Cantrips like ray of frost and fire bolt are far from overpowered. I don't see how this change is necessary or adds anything to the game of value. Ignoring the question of why you think this is necessary, I'd call this change sloppy. It does not sound fun. You either have adventuring days where your casters run out of spell slots and are forced to use their now totally useless cantrips*, or you have adventuring days where they get plenty of rest and have more spells to throw around and never use cantrips at all. Neither option sounds particularly engaging.
*Yes, at 11 a cantrip that does 1d8 damage is useless, you'd be better off providing the help action to the fighter.