To my eye a spell caster is in a better position that an weapon wielder. First, for many spells there's still an effect on a successful save such as half damage. Second is the nature of the save system. With only two trained saves, even with good ability scores that don't align with those saves a foe has at best 2-4 decent saves. A caster can and should go after the poor saves. Where a +3 from ability and then +2-6 more for proficiency is matched up against a +0 or +1 save - a much better chance to succeed than an attack with a +3 STR or DEX.
But that's just a digression, I had an idea about the ASI that I wanted to suggest. What if the modifiers from ability scores changed slower as you go up? Making somnething up off the top of my head, say an 11 is +1, 13 = +2, 16 = +3, 19 is +4 and 23 is +5. Change the cap to 23 as well, and some monsters will need to be updates.
So going up in ability scores some makes sense, but once you are high it will take more than one ASI and at that point feats become a better deal. So you can push forward for the biggest bonus but at a high opportunity cost, or grab feats. That might introduce some variation. (Again, those are just sample numbers I picked out of the air, don't think of them are rigorous.)