Someone made a comment about wizards in 5e being hobbled by attunement slots and concentration, but I can't find it...
I think that this is a necessary step to reduce the linear fighter, quadratic caster problem.
I think something like concentration is necessary. Concentration as-is takes things a little bit too far, I think, but the error in that direction is much smaller than the error in the other direction used to be.
I have a memory of reading a blog from a D&D designer (pre-5e - it might have been Rich Baker, but I'm not sure) who floated an idea about casters being able to maintain two spells at once, one offensive and one defensive/enhancing. That sounds roughly right to me. That would still keep wizards from being invisible, flying, hasted, and stoneskinned, but would allow them to have fly and hold person up at once.