It's also worth nothing that casting in melee being a Really Bad Idea was very much a feature of the classic game. Interrupting spellcasting goes all the way back, and wasn't done away with entirely until 4e. Even in 4e, most spells (ie Range & Area spells) provoked AoOs, even though being hit didn't prevent the spell from going off unless the caster was dropped.
5e is unique in giving casters a free ride this way.
Also, if you do go that route, why can’t someone then interrupt the fighter, trying to distract them from landing their hit?
Someone fighting in melee is already protecting himself from attacks, as a matter of course, and resolving an attack on your turn already represents making every effort to hit the other guy. A melee 'attack' also represents a whole round's worth of blows, even if it's only 1 attack (or 4 or 8) resolved on your turn. Most of them are 'interrupted' in some sense, or they'd all hit.
Even so, protection style, the Help action, and Sentinel can all already be used to interfere.
In contrast, loosing an arrow is loosing one arrow, and you could Ready to react to it...
Or the rogue lining up that perfect shot with a crossbow?
Already has Disadvantage for trying to make a ranged weapon attack while in melee. You can ready to attack him when he does, but pulling a trigger on a crossbow is prettymuch instantaneous, 'interrupting' seems unlikely.
Still, it'd be cool to have a more granular rule for Ready in general, maybe contested DEX checks or something, to give the Readied character a chance to resolve his action first. (Note that, applied litterally to spells, that'd just mean the caster takes damage, then, if he lives, goes ahead and casts.)