Furthermore, someone who claims that "my fun does come from effectively solving challenges" (I lean towards this view myself) has no grounds to claim that the mechanics of the game (concentration) is "limiting his fun", because solving the game within the constraints of the rules as written is the source of the fun! (My players are more the "glorious and reckless" sorts so I don't get to indulge this side of me very often unless I'm playing in someone else's game, which is why Celtavian's dragon challenge sounds like fun.)
So someone who claims that the game is limiting his fun by not enabling melee fighters to match ranged fighters must be speaking from an aesthetic standpoint, as in, "I think the game would be more fun if melee weapons were better." Which is fine. It's no different from the way I think the game is more fun with spell points than spell slots, so I've adopted the DMG variant--because it's nice when the physics of the game world reflect the reality you're interested in playing.
It's a pretty clear issue created by the game mechanics. A melee martial cannot bring his abilities to bear against a dragon without fly. It has reach. It moves at 80 feet a move. I'm not sure how you play dragons, so experience can differ. We play dragons taking opportunity attacks. The calculation is simple. One opportunity attack is far better than a full round of attacks. So it will fly in do a full attack, fly away to cover or darkness (80 feet brings it out of most creatures dark vision range). It will do this round after round using lair actions to further damage the party. It opens with the breath weapon and uses it every time it is available strafing the party as often as it can. It can usually kill a party faster than you can burn down its legendary resistance.
The claim you can burn down Legendary Resistance before the dragon kills a five person party seems strange to me as well. I've fought these things. A 100 hit points of damage in a round is not at all unusual. Often a large portion of that opening damage is AoE. It's nearly impossible to heal it up quickly on more than one character. You don't have a lot of time to test spells on the dragon. Even if its Dex save is low, it would still take three rounds to burn Legendary Resistance if it misses every save and chooses to use LR. If it makes any of those saves, you lost another round trying to burn down LR and doing perhaps no damage in the process while it is trying to kill you with full attacks and breath weapons. The only thing it is losing is the Legendary actions for attacks it might take if it stayed in melee. If one of you are close to dead, it may stay in melee to finish the job. If it is able to eliminate one or two party members while you are trying to burn down its LR, it wins, most likely an easy fight.
We used to deal with some of this with the ready action resetting initiative. Now the ready action doesn't change initiative and also takes a reaction. If you ready an action to attack the dragon when he closes on you or another character, you only get one attack and no AoO. Extra Attacks only applies to your action on your regular turn. So you only get one attack per turn whether you ready or take an AoO. If you ready an action to cast a spell, it takes your concentration slot. Thus you can't ready a spell or you take your concentration slot until you cast it. That makes using ready actions to cast attack spells if the dragon clears cover and comes into vision and spell range tricky.
A lot of little mechanics make dragons and powerful fliers extremely hard to fight, especially so if you are melee martial heavy and unwilling to use a fly spell. You could try all the tactics you list to try and solve the problem, then discover that the easiest and most sure method is cast fly on the melee martials. You're still back to square one. Your most effective option is fly. It's not fun after the fifth time you've solved the problem in exactly the same way because every other way is far and away less effective and efficient. That has nothing to do with "fun solving challenges". Fun solving challenges is coming with other tactics equally effective to casting fly, not casting the same spell to solve the same challenge time after time.
Let's say you do beat the dragon in one combat of five using an alternative to fly, but your party dies the other four times. Then let's say you cast fly and win the battle five of five times. Would you consider that fun solving the problem within the parameters of the rules? If the rules create a situation where there is only one highly effective way to solve the problem and all the other methods have a relatively high chance of failure, is that fun to a problem solver like yourself?