Casters had 3 targets for saves; the cost to defend against a spell was 3x larger.To be fair, high spell counts for high level spellcasters was nothing new. 1e/2e had it as well.
The issue of spell save DCs was an interesting one. While it's true that the chances of a similar (or higher) level foe failing the save went down in 1e/2e, there were elements set up in the 3e system that were set up to do something similar for strong saves and common bonuses you could get to your saves were cheaper than most bonuses the caster could get to boost your save DCs. What that ran into was optimization-mania. Most likely targets weren't going to focus on just boosting their saves while spellcasters had all sorts of incentive to invest in their spellcasting stat. I figure this sort of issue is one of the reasons 5e caps the stats. It no longer can skew way out of proportion.
And the "natural scaling" of both was about the same. Ie, the number of ways to add +X was about the same, and the superlinear cost increases where on the same scale.
Getting +4 to your constitution was roughly as hard as getting +4 to your casting stat. Getting a +4 fortitude save bonus was similar in price to getting a +4 to your save DCs.
Your "strong" saves went up at level/2; spell slot levels went up at about level/2.
Your "weak" saves went up level/3; this gives casters a level/6 advantage if they target a weak save.
So you have to invest roughly as much keeping your strong save to match spell save DCs, and more to get your weak saves matching spell save DCs. And then more again because you have 2 weak saves.
And if you do all of this? You are no more effective at doing stuff. You just managed to tread water at blocking some subset of what spellcasters can do to you.
Meanwhile, "no save" spells continue to grow in number, decrease in cost. So even if you do invest enough to tread water against spell saves DCs? You still get to "no save" and suck.
Meanwhile, in AD&D and earlier, each spell cast by a spellcaster had reduced chances of landing against tougher foes.
A level 21+ warrior has a 75%-90% save chance against various effects, with the more severe a better save chance. A level 1 warrior has a 5%-25% save chance.
It requires 1.05 to 1.3 attempts to land a spell on an even level foe at level 1 in AD&D2e, and 4 to 10 attempts to land a spell at level 21+. Even if we take the easiest save at level 1 and hardest at level 21+, this is a 3 fold reduction in spells landing.
To reduce offensive magic by this amount, even if the defender is "treading water" on saving throws, you'd have to:
1. Your number of spell slots at level 20 is 1/3 as many. Just strip piles of slots out. Reduce bonus spell slots. Etc.
2. Good saves progress at +5/4 (so +27 at level 20). Bad saves progress at +1 (so +20 at level 20). At level 6 and 11 and 16, a spellcaster can spend 1/2/3 extra rounds to increase spell save DCs by 5/10/15 points.
A spellcaster spending 4 rounds on a spell has +15 save DC; the target has +14 to +15 bonus over the previous saving throw tables. This acts like a "1/4 as many spells you cast actually land on target as they did at level 1" factor.
And then do a pass to weaken a pile of "no save and suck" spells, and probably boost spell blaster damage without crazy charop (3e high-HP made blasting ineffective).