In your hypothetical, that version of D&D would likely be *balanced* (at least designed) for infinite reactions. Enemies' stats and CR would be modified based on the expectations of taking multiple reactions; certain abilities like Counterspell, Shield, and Protection Fighting Style would probably have text making them more restrictive and/or less powerful; etc.
Part of what I'm saying is that I don't believe the balance is quite so delicate by far.
Another part of what I'm saying is that I think the reaction system design isn't very good. I do not favor the player deciding to use this bonus for that roll and so on.
I do, however, favor the idea of forgoing your action to ready an action, whether that be a readied action against a potential attack or a readied action against an incoming spell. But I don't believe those readied actions should be classified as "reactions." Rather, I think they should be a replacement of the character's action, meaning if the readied condition does not happen... the character has effectively abandoned their action for the round.
