Concentration serves two very different purposes at the same time.
You've discovered the first: the way it prevents stacking. This is good for the game (and this opinion is borne from experience from d20/3e where the buffing precombat ritual destroyed the game)
The second is to add a measure of vulnerability to casters: the way you risk losing a concentration spell if you suffer damage.
The problem is that some (not all, but a few) defensive spells really need the first part (to prevent stacking) but aren't useful because of the second.
For instance, a spell that halves damage taken (Stoneskin) is a spell that's explicitly about entering combat. And yet, as soon as you take the damage the spell was designed to help you endure, you must start rolling Concentration checks.
That's just messed up - you're so much better served by instead picking a spell that removes you from the situation. If you can't rely on your self-defensive buff to do its job as advertised, its value and utility just plummets.
That still doesn't really make it a problem. IIRC Concentration checks are either 10 or half the damage dealt, whichever is higher. With stoneskin up, you would need to take a hit of 44 pre-mitigation to have a DC 11 concentration check. The number of enemies who can reliably hit you for that outside of big ticket spells or abilities (like a dragon's breath) are very rare. Additionally, in a game that allows feats I can't imagine a caster that doesn't take Warcaster for advantage on concentration saves, so it's further mitigated.