I think if you just look at a list of spells regardless of class and which ones have concentration and which don't, you'll get a misleading impression of how the mechanic works. Having played in games with a few different spell casters now, it seems that some classes have concentration heavy spell lists by design as a way of balancing them.I do think the designers of 5E went a little overboard applying the concentration mechanic. Some spells are pretty much never going to be taken because they use up your concentration slot. Other spells require you to get into melee, and that's rough because then you get hit and stop concentrating. Then there are completely awesome spells like spiritual weapon that for some bizarre reason don't require concentration. It totally doesn't make any sense to me.
I'm not sure what your definition of 'fiddly' is, but your houserules are what I would describe as fiddly, not the concentration rules.My table finds rolling concentration checks for damage too fiddly for us. So we changed that part, we only make concentration checks when the attack exceeds ac by 5 or more or if the blow bloodied the PC (we kept the concept, I throw a slight penalty on them when they are bloodied, this encourages the players to avoid playing whack a mole, they are less effective if they are low on hp)
It might be more things to track for other tables, but we already do magnitude of success even with attacks, right on ac = half dmg, beat ac by 5 = max dmg, beat ac by 10 = crit, so we are just tying another thing to the magnitude of success
Never had too many issues with it in my games really with the stacking. Unless the GM at high level was preparing an evil lich or wizard. Even then all the buffing in the world could not stand up to a dispell magic or antimagic field, or mordenkainen's disjunction spell.I don't understand the problem. Concentration *is* a fix; the problem was the stacking buffs/debuffs that trivialized the contributions that non-casters made.