It seems that 5e has created a new variant of this same problem, that a certain category of fights are dull because the optimal tactics are both boring and obvious once you have figured them out. Obviously there are ways to mitigate the issue, like drop Boots of Flying into the loot; but that is an implicit admission that Celtavian's point is correct.
The general idea of the concentration restriction seems like a great thing to me. But it may be too all or nothing and too inflexible for making the game be as fun as possible.
I think that the Concentration "only one such spell" mechanic went too far overboard in the "get rid of 3E multi-buff".
The four other new pillars of magic: save every round, and concentration can be broken, and most buffs last for an encounter at most, and higher level spells are at a premium, really are sufficient.
In fact, the 1E through 3E games were played for decades without any of these rules and not everyone complained about them. Some people just had fun. Some people complained, but it wasn't everyone. Some of us just had a blast playing the game and didn't whine about Codzilla.
Dragons are intelligent. I think that it makes total sense for a Dragon to send every single one of his henchmen at whomever is casting a Fly spell and just kill that PC (and I don't mean make him unconscious, kill him). That prevents the tactic of the flying melee PC and the Dragon now rules the sky again. Even if the Dragon has to come down and kill the caster itself. Fly down, kill the PC, fly back up all in the same round.
So far in all of these posts about super tough encounters because the PCs are super optimized, I have not once heard how the DM kicked the snot out of the party with a tough encounter run intelligently. I just hear how great the party is because they work as a team, control the battlefield, have these really optimized PCs, etc. I haven't read where the DM has one monster hold off the PC fighter by dodging, how a group of monsters gang tackle the melee guy with multiple shoves and grapples, how the NPCs cast wall spells to split up the party, or come in at them from multiple directions, or force the PCs to come through a choke point, or cast Darkness with area effect spells. The DM has a ton of options to challenge the PCs.
The problem that I have as a DM is that the players are playing PCs that they are constantly improving. The players are learning how to get better tactics and synergize their spells and abilities to the point that they become a very effective team. The monsters, on the other hand, typically only get one shot at it and do not get "opportunities to improve".
There are only two ways that I see to overcome this. Throw tougher foes at the PCs. Or. Pre-plan encounters with specific tactics, environments, and foes that make problems for the PCs. The former is easier to do. The latter takes a lot more effort and I must admit that I do not always have the time to put in that level of thought and effort.