Gaining +10 base to hit in 10 levels isn't a little. Going from 25% accuracy to 75% is a lot.
Not really.
10th level monsters according to 1e. Assuming a +3 weapon and a 17 strength.
Demon Princes: -5 to -8 AC - That fighter would need a 14-17 or higher to hit. That's a 20%-35% chance of hitting.
Arch Devils: -2 to -7 - which equates to a 25%-50% chance to hit.
Iron Golem: AC 3 - This one is actually 75%!
Lich: AC 0 - 60%.
Elder Titan: AC -3 - 45%.
Vampire: AC 1: 65%.
So only one has a 75% accuracy rate and it drops as low as 20% accuracy.
Even AD&D Priest and Rogue accuracy increases faster than 5e.
So what. It still isn't "ballooning." The cleric is looking at a 65% accuracy(for the golem) down to 10%. And the thief is 55% down all the way to 0%(can't remember if a 20 always hits in 1e). That's hardly intense accuracy.
As for the casters, I thought I didn't have to remind the original "raiding increasing number": spell slots.
I'm not sure what that means, but mid to high level spells were pretty easy to interrupt.
Edit: I also give both the cleric and the rogue a 17 strength like the fighter, but that almost never happened since the high stat was put into wisdom for clerics and dexterity for thieves. Also, I gave the PCs magic items, but neglected to give the monsters any and almost all of those listed would have and use them, so there is a good chance those ACs would be even lower, further reducing accuracy.