Concentration isn't the be all and end all of balancing spells. In B/X for example a lot of the problem spells just don't exist, and buff spells in AD&D were rare and mostly a +1 bonus (aid, bless, prayer). Buff spells are even more rare in B/X.If you're saying that 5E's implementation of Concentration isn't perfect I agree.
But this discussion isn't about the fine-tuning of 5E. Going back to the overall issue, we were discussing the fact that Concentration-or-something is what finally fixed d20. That Paizo is making a mistake if they give us yet another jumble of 3rd edition bits and pieces with far too few constraints on casters.
Then I made the point that adding a Concentration mechanic (again, or something else) isn't enough. You must also ensure almost every spell is bound by that restriction, or it is meaningless. All that would accomplish is allowing a narrow set of spell choices to dominate, and utterly relegate the vast majority of alternatives to irrelevance.
In that light, I would submit that yes, while 5E might not be perfect, they at least took a giant leap towards actually fixing things compared to the useless refaffing that was 3.5 and "3.75".
By that I mean that Pathfinder might have enormous value because it allowed d20 to live on. But coming close to actually fixing LFQW? (Or even fixing the problems they bragged about fixing) Nope. To me, 3.0, 3,5 and PF are same same but different. Sure 3.0 might have even more imbalances than later models (details about monster damage resistance, psychic combat, what else?), but compared to 5E they're all equally outdated.
If nothing else the existence of 5E should mean no publisher will ever publish a D&D:ish game again without its fundamental upgrades to the magic framework. But the PF2 playtest isn't exactly filling me with confidence Paizo have learned the lessons taught by 5E.
Or even that they have tried to learn them...
Spell design (even 4E tried that in a way) and the way they work with things like saves and spell resistance are other factors as well.