You know the tv show Avatar: the Last Airbender had that problem. By the end of the first season the party consisted of three of the most powerful benders (read wizard/elemental monks) in the world, plus Sokka, who was a teenage guy. He openly wondered about his role in the group and more than one episode was devoted to the question. They actually essentially went on a side quest to get him levels in a prestige class at one point. The actual answer was that he was indispensible due to party dynamics, but that is a writers solution and would be table dependent in play.
I'm not familiar with the source material you're citing. I think M Night (maybe) did a movie (?), but I didn't see it. Regardless, I think I know what you're intending as your salient point; indispensability due to party dynamics means Fighter's role is always legitimized. However, I didn't find that to be the case when GMing late mid/high level play in 2e (even though I think the 2e C&T Fighter was second to only the 4e Fighter in its awesomeness) and definitely not in 3.x (by far the worst iteration of Fighter - a truly terrible, awful class and nerfed horribly with respect to its predecessors). In 1e UA and 4e, yes, the Fighter was a boon and his relevance endured (especially 4e).
The upshot of this is that the only reason to play a mundane character is because you want to play Sokka, or Samwise Gamgee. You are the voice of normality in a party of demi-gods, and your role in to ground the party in reality and remind them that not everyone is a caster/demigod/agent of fate. If you don't want to play that character, why did you? Conversely if someone does want to be Boromir in the same party as Gandalf, Legolas and Aragorn, why should he be compelled to play Hercules instead?
They can do this. They just need to play a lower level than the rest of the gang. While Boromir was an accomplished warrior, he wasn't even close to being in the league of Gimli or Legolas (all three of them Fighters if I had to put them in D&D nomenclature).
Legolas is an interesting character here. The source of his otherworldly martial ability wasn't supernatural. Further, if Gandalf is only level 6ish, I certainly wouldn't classify him anywhere near Epic tier (level 17 +). However, in the fiction, in all ways his abilities make the 5e Champion look like an absolute chump. You cannot get Legolas out of the task resolution system and bounded accuracy of 5e. Legolas could lay low dozens and dozens of orcs (with bow or blade) and float around the battlefield like a wraith. He could bring all of that big damn hero, crazy athleticism (with pretty much 100 % efficacy) outside of combat for noncombat conflict resolution. The Champion couldn't dream of that kind of stuff. Even at double his level.
With respect to the other means to have the Boromirs and Samwise Gamgees in a group of superior warriors, it doesn't just have be done via the level system (though that is the way it should be done for an agenda that is seeking process-sim). It can be done beautifully with metagame mechanics. To name a few, Fate and MHRP can do it via their infrastructure, BtVS can do it by giving the Scoobies plot points, 4e can do it via princess built Warlords or Companion Characters.
Now in 5e they have a multi pronged fix to the problem.
The power of casters has been severly reigned in due to fewer high level slots, reduced save DCs and most importantly the concentration rules.
I've yet to GM more than the early playtests (and I expect I won't ever be GMing it unless some of the modules turns it into a better one-off dungeon crawl than 1e or RC). However, the system seems so familiar to me and I've run so very many AD&D 2e and 3.x games (this system basically looks like AD&D 3e and I'm certain it plays like it), that extrapolation seems trivial.
I'm reluctant to buy-in to 5e's multi-pronged approach to balance at mid 2nd tier through Epic. The vast majority of Conctration checks will be in the vicinity of 10. This is trivially worked around via the Warcaster feat, the Resilient Feat, Transmuter's Stone, potions or stuff to give you resistance or having Shield and Mage Armor. A level 10 High Elf Wizard with any combination thereof (or even just 14 Con and Warcaster will be passing typical Concentration checks at a rate approaching mechanical irrelevance).
The reduction of DCs is accompanied by the 3.x save paradigm. This, of course, means that creatures you are fighting are going to have outrageously poor saves in 2 of the 3 big saves. In terms of spell proliferation being reined in, I'm not sure I see it. Take the below Basic Set, Evocation specced, High Elf Wizard at level 10 (halfway through tier 2). Again, this is missing some big guns from the PHB. Just this guy below is ridiculously powerful.
Blasty McBlasterson - Level 10, High Elf Evocation Wizard.
1) 20 Int yields 5 + 10 = 15 spells per Long Rest.
2) Known spells just from leveling (no spellbook poaching or found spells)
5 cantrips
8 first level spells
4 second level spells
4 third level spells
4 fourth level spells
4 fifth level spells
3) Able to cast spells as rituals
4) 5 spell levels recovered with AR
5) ES should mean there will be more evocation spells in spellbook
6) SS = 1 + spell lvl allies automatically pass their save so no friendly fire
7) PC = enemies still take 1/2 damage on successful save.
8) EE = + 5 damage to cantrips so you have more flexibility to save the majority of your 15 spell slots (of which you can get 5 more spell levels back from AR) for utility. Just the below Cantrip load-out is awesome (and makes Gandalf weep)
Acid Splash for AoE (1/2 damage on save, no friendly fire, + 5 damage)
Firebolt for single target (1/2 damage on save, enormous range, + 5 damage)
Minor Illusion for tricksey
Mage Hand and Prestidigitation for Fantasia
9) With those buffed Cantrips and AR I've got enormous versatility to pick a load-out heavy on any of the ridiculously awesome spells (encounter enders or nerfers to the point of obseletion + recon/divination master + Face + infiltration + massive defense by way of mobility/escape contingencies).
Arcane Eye
Passwall
Wall of Stone
Greater Invisibility
Major Image
Fly
Suggestion
Levitate
Web
Hold Person
Invisibility
Sleep
Disguise Self
Charm Person
Mage Armor
Shield
Protection from Energy
Dominate Person
Wall of FIre
10) I'm not seeing a lack of flexibility. I'm seeing flexible and filled with combat beat-down/control.
