Given that my preferred edition is 1e, I don't see any problem with spells simply having a duration....
As the DM:
So if I were fixing this in a vacuum I'd just drop the concentration mechanic.
But I'm not fixing it in a vacuum. So I just let the players decide if they wanted to keep it as is or change it somehow.
For the time being they've chosen to play it as is. We'll revisit the question next campaign.
And as the DM my monsters are not hindered by this if I don't want them to be because monsters don't have to work exactly like PCs.
As a player? I'll let the table/DM know my preferences. Then I'll roll with whatever the group has decided.
It's just a question of do I solve the problem using x aproach or y.
I'm a long time runner of 1E and other OSR systems. I hate to ask (because it sounds condescending) but have you played/DMed high level casters in 3.x or 5e?
Sure, 1E doesn't have the Concentration mechanic, but here are limiting factors to spellcaster power in 1E that 3.x did away with:
1. If you cast a spell, you can't move that round. At all. Sure, maybe your ring of invisibility will pop back up, but with your spell you've given the enemy your location. In 3.x, you cast, you move, you're invisible again.
2. You had no way to improve the DC of the saving throws against your spells; in 3.x you had several ways to game your spell DC.
3.
Every class and
every monster decreased (improved)
ALL of their saves as they leveled/had higher hit dice. The fighter wasn't a liability to a measly Charm Person spell, but could laugh off "save or die" effects and then slice the spell caster in half. In 3.x and 5e, a spellcaster has the repertoire to choose spells with a variety of ability saves and tailor their cast to the target, sometimes ensuring that it will fail its save. (At least 5e, while keeping some 'save or suck' spells, gives the power low save critters a chance to save each round.)
4. You couldn't pick your spells: each level you learned ONE random spell. You had to find the rest as treasure, purchase them, etc. Oh, your first level spell is Push. Yeah, you can be creative with it, but you know everyone wants Sleep. In 3.x, wizards get to choose, and they get 2 of them.
5. Spell memorization times. At higher levels, it can take days or even a week for a wizard to re-memorize their spells. You had to _very_ carefully conserve your spell slots. Want to re-memorize in the dungeon? Enjoy a zillion random encounter chances. It takes an hour in 3.x if I recall.
6. Magic Item creation. It was nearly impossible to create magic items besides potions and scrolls, considering that Permanency, an 8th level that requires a whole POINT of Constitution, was necessary. (Yep, according to the 6th level "Enchant an Item" spell, no magic would be permanent unless Permanency was cast). 3.x let you be a veritable custom to order factory of magic items.
I won't even say this list was all-inclusive. Your hit points couldn't benefit from above a 14 constitution unless you were a fighter, for instance. There were so many ways that spell casters, for all that they did have Save or Die spells, were less powerful and more fragile than in 3.x
5e keeps many of the 3.x changes, but keeps spell casters somewhat in check with Concentration.