I fail to see how you conclude that such drastic measures are required.
In setting, it is rare to find that most of your time is spent in combat where such reactions would be worrying. And often enough, the party is either in control of or aware of when combat will take place.
Like..it's not typically the monsters that are kicking the doors in on the PCs.
Even assuming that spell interruption rules apply globally to all PCs and monsters (which isn't a necessary implementation), they would only apply specifically in combat and then only specifically when the caster is within melee range of a monster (assuming existing reaction attack parameters).
So the usefulness of spells would be unchanged outside of combat (where they offer leagues of capability beyond what a martial can do), and unchanged in combat at range (where they also offer leagues of options beyond what a martial can do).
For my money, this is most of the time spent in D&D already. And the remaining circumstance, being within melee range of a monster, isn't a circumstance most casters were seeking out anyway.
And note again that this would be assuming that we give the same reaction to every character everywhere. We don't have to do that. We can make it a PC option only, we can give it to some monsters and not others. We can vary the trigger parameters of the reaction and the execution methodology. Like, nothing is set in stone here.
In summary, I think you are overreacting to a potential (and specific) change to mechanics that only apply in a niche circumstance, a circumstance that is already avoided, and that already has tools available to mitigate the risks when it cannot be avoided.