I play a wizard (currently 9th level) in tabletop D&D 5e and without Concentration to limit her, she'd be almost unstoppable. Concentration is a game-balance mechanism. It might not seem to be at lower levels, but when you get more spell casting slots and access to more spells it becomes important.
Since D&D was around casters always started pretty weak and got stronger later on, read some novels on Elmister, Khelben, Gromph, Mordenkainen, to list just a few. Rulewise you can still stop them. The other classes get better at mitigating the effects of spells too. With spells like "wish" in the pool such things are to be expected.
I do not say that i do not understand the logic or even the plan on the concentration mechanic. On some spells i think it fits pretty well. But compare "hold" and "sleep":
As of now you PUT people to sleep but have to concentrate to KEEP them paralyzed. OK. Works.
But could you not also say you paralyze people for a TIME (as has been) but you need to concentrate on the sleep spell to KEEP them sleeping (mostly during a very loud combat situation which would wake up even a drunken dwarf). Against both spells victims get a save but against hold there are much more ways to break it.
In case of hold, maybe give an extra save each time the victim takes damage.
