This boils down to the Ship of Theseus argument. I disagree with this notion; fundamentally altering the game mechanics of how they were intended to work is not 5e anymore.
It might seem like a non invasive change, since you are not altering the HP system, spell slots and resource economy directly but essentially you are doing just that. You alter the core mechanics of the game.
Granted many DMs and Players never cared about this in the first place, so I can see where you are coming from. I was once like that as well, until I ran the game it was designed to be. That's when I ran 5e, and not a homebrew combat system.