I'm going to agree with DND_Reborn. A lot of things were different then so there's no perfect comparison Yes 2e & 3.x were different but getting attacked while casting a full round cast or getting popped with an AoO because your casting provoked an AoO were important threats that 5e simply does away with. The risk was so extreme that casters did everything they could to avoid being at risk of near melee range opponents & there was interplay between caster/noncaster to manage that risk as a team. A weak mook may only take 1-2 hits & be of no real threat but 2-3 of them wandering around chasing a caster was an unacceptable risk that pushed the group to split up dealing with them and the big guys rather than just ignoring them & dogpiling the biguy.That is highly debatable. They do have some disadvantages (lack of caster level scaling), but OTOH AD&D had much easier spell interruption (you rolled initiative each round and anyone who beat you on a given round could interrupt your spell - and if they managed to inflict a single point of damage before your turn the spell was gone), and if by some miracle you did manage to get a spell off at higher levels saves scaled in the opposite direction in AD&D to 5e; high-level AD&D characters almost always made their saves.
3e got rid of most of that, without adding anything really to replace it, for that on other reasons it is fairly uncontroversial to say that 5e casters are weaker than 3.x and PF1 casters. OTOH, caster supremacy and quadratic wizardry in 4e were negligible to non-existent, so 5e did not nerf casters, it buffed them.
All that said, I actually like the idea in the OP. While it is obviously a buff to classes that probably do not need buffing, my feeling (as someone who does not claim to be a 5e expert) is that it is not a huge deal. More of a quality-of-life improvement than a big increase in the power ceiling. If you gave martials something decent to compensate, you might end up at a more balanced point overall than RAW.
I am considering doing something similar in my personal heartbreaker, should I ever get around to actually writing the thing (which I probably won't - fiddling around with bits and pieces of mechanics is fun, but actually forming them into a coherent whole is a lot of effort)!
With that loss it became easier to justify the kind of problems that thishouserule addresses but the loss is not an improvement for casters