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.
I mean, that's fair. Your middle statement, at least, is purely an opinion about your preferred gameplay style and thus doesn't really need evidence or reasons to be valid.
Your first statement...could use some proof, but I'll certainly concede that D&D isn't perfectly balanced. The problem is that there are a LOT of abilities in 5e that ask for some immediate player decision (both things that expend your capital-R-Reaction and abilities that allow you to add/subtract a bonus mid-attack, etc), and the game was designed around these abilities and the fact that characters have to choose between using 1 per turn max. If you change them (make them pre-actions or make them unlimited), then every single one of these abilities will have an altered power level, the effects of which are probably too large to determine just by thinking about it. You'd have to create such a game and then actually playtest it...which I suppose is what EA is for, but making such a drastic change seems not worth it to me.
There is already a Ready Action in 5e which works exactly as you describe. It is part of the PHB and afaik Larian hasn't commented saying they *won't* implement it, so theoretically it will be present in some form. But we obviously should keep expressing the opinion that we want it in the game to pressure them!
