Allow me to explain:
From a minutia perspective, yes, the fighter cannot fly unassisted by magic or DM fiat. However, we are not playing a game of isolated challenges without any context.
I gave an answer as ridiculous as the challenge: to simply float in mid-air. The only reason you've come up with this challenge is to say that a caster can fly. But a fighter can do anything the caster can do while they're flying through mundane means. Like you said, it could be for scouting, intimidation, or having to grab something from above. But a fighter can scout, intimidate, or grab something without having to cast a fly spell.
Magic is not meant to be this catch-all get-out-of-jail-free card. There are flaws to magic use.
Magic's primary use outside of combat is not to solve impossible problems, it's to solve problems quicker and more efficiently than through mundane means. At the expense of combat ability.
Flying is an advantage in scouting except you can be easily seen. It also means anything that hits you threatens your concentration and you could start plummeting to your 20d6 fall or waste another spell gently floating down just to be constantly being attacked anyways. You can't fly and be invisible without an external item or spellcaster, so if you're flying, you're going to be noticed.
Again, magic is useful. But whatever you're trying to accomplish with it can be done through mundane means.
Except, spellcasters are also typically better in combat too.
I mean, sure, the Fighter can kill ten goblins with his sword, if he is 20th level and burns a chunk of his resources, he might even possibly be able to do it in 2 rounds.
Fireball does it in a single action, and by 20th level isn't even breaking a sweat for the Wizard.
Not to say there are not flaws to magic use, mostly because of concentration, but a wizard can solve combat problems a fighter simply cannot, like denying actions to the enemy, or protecting a crowd of civilians, or preventing a crowd of enemies from escaping. Along with being nearly as deadly if not deadlier, and solving out of combat problems too.
Again, these are all context-less challenges. Why do I even need to get to the BBEG? What's the purpose.
I can escape a wall of force through mundane means, it's called waiting.
Magical problems exist but there's always a mundane solution in actual games.
The problem is not to get into the BBEG's castle, it's to kill the BBEG. Rather than going into his home turf, you could convince him to come down and fight you.
Would that work? Who knows. There isn't any real adventuring context. We're just plopped in a empty void scenario like a bad tutorial level and expected to solve the problem using only one obvious method.
"Waiting" isn't a solution. It is losing.
Sure, they can't keep up the wall of force forever, but they know that to, so they are only trying to delay you until they can accomplish a goal. And your solution is to be delayed, because you have nothing that can do anything else.
And gave up toughness, strength/speed, essentially mundane combat. People act like combat doesn’t make a difference isn’t a massive part of the game. You may think your handful of high level spells are worth that... I would say it’s a tough call.
Incidentally D&d was always intended as a cooperative team game not a competition.
The 20th level fighter (champion) is effectively regenerating, attacking 4 times faster than a normal person when they aren’t trying, 8 times faster when they are; dealing a critical hit, approx twice every three rounds, fighting with any weapon, they’re also slamming their shield in front of enemy attacks to protect their friends, or tripping people with a glaive and stopping them in their tracks.
When the wizard turns into a dragon, the fighter hits it and it turns back into a wizard. The fighter feels fresh as a daisy and the wizard has lost their most powerful spell.
Regenerating to half health, as a note.
And, an adult red dragon with warcaster literally can't fail a concentration check of less than 15, which means 30 damage in a single strike. With advantage and a +13, I'd give that wizard decent odds of needing to take over 44 points of damage in a single blow to lose concentration.
Meaning the fighter needs to just get through all 260 hp of the dragon, who is first of all going to open with a Breath weapon for around 63 damage, then rip and tear for 58 per turn... and when he turns back into a wizard, he just casts another spell like power word stun, or disintegrate or any number of nasty effects, with full health, because the fighter first fought a dragon.
And again, the idea that the wizard is losing out on combat, when they are actually more powerful in and out of combat, boggles me.
What's it matter where it comes from so long as they can do it?
Because casters get loot and treasure too.
If the fighter needs to get a carpet of flying to start flying (something the wizard could already do) and the wizard gets a staff of fire to give them even more combat spells, the fighter isn't catching up. They are falling behind.