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.
It depends largely on if you are talking 2E or 1E. In 1E, initiative and spell-casting worked differently. In 2E, with the addition of casting times and speed factors, it was a bit more of an issue, but not much.
Personally, IME casters in AD&D didn't have the issues you're talking of because they avoided melee as much as possible, and like the advice in the DMG, resorted to wands, staves, and rods while in melee, which could not be disrupted.
Yes, the saves at higher level were more likely made (part of what bothers me about 5E--look at the most recent Vecna battle and how often the PCs
failed those saves! It was rare when they made one...). But for many spells in AD&D, making the save was more important due to SoD effects. You made saves more often, but when you failed them the impact was often drastic!
Like other changes... AD&D hitting was hard, but damage mattered more. Saves were easier, but the effects harsher. In 5E it is reversed. Hitting is easy, but damage impact is lessened due to HP bloat. Saving is harder, but effects aren't as extreme and saves are often repeated until made.
Concentration is on
way too many spells IMO, although making the check often isn't too bad if you get hit. But, it prevents the "combos" so many people dread which made casters so powerful in AD&D. Much of that power has been nerfed, all things considered.
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 like the idea, of course, but there are ramifications which I am still thinking about. Also, I don't know about the martials. I don't see this as a power boost to casters, more about bringing lower levels spells up a bit to keep them viable at higher levels in the game.