Tony Vargas
Legend
First, of course, 5e doesn't use formal role, and doesn't seem to depend on niche protection, per se (that is, there's very little in the way of vital functional contributions that can be made by only one class).If that is the issue - that players in a "play circle" generate PCs blindly, and then turn up, and so may not have every class or "role" covered, and hence can't be guaranteed to have a full suite of spellcasting or similar specialist solutions available - then what is the answer?
Where the problem comes in is matching broad concept to contribution. If you want to play 'an arcane caster' (not much detail, but hey, it's just an example), you can certainly wiggle that basic concept/preference around to contributing DPR (a Warlock or EK), contributing support (Bard for healing, any of the others for buffing), contributing lockdown control (Bard or Wizard), contributing crowd control (Wizard/Sorcerer), contributing out of combat (Wizard, Bard), checks (bard), etc, etc. No problem. If you want to play a character who doesn't cast spells in combat you could still choose a Monk, a Barbarian or a Paladin (just use your slots to smite, lay on hands isn't technically a spell) in addition to a non-AT/EK Rogue/Fighter - and that can cover DPR lavishly (all those classes can do some good DPR), and support, modestly (Paladin), and checks (Rogue), and I'm sure Monks are good for something, too. But your range of contributions has lost some depth. Say you don't want any sort of magic at all in your concept. You're down to Berserker, Champion, BM, Thief & Assassin. Possibly contributions are DPR (all) and checks (Thief & Assassin).
So as long as at least some of you want to play casters, you can probably negotiate needed contributions and cover all your bases. But the all-'martial' party is off the table.
There are solutions, and there are work-arounds. 'Play a class you don't want to,' is not a solution to the problem of a party lacking a vital contribution like healing. A wider range of more appealing concepts able to provide the vital contribution is a solution. You can see that happening as the game evolved with the Cleric, specifically. Healing went from Cleric-exclusive in 0e, the Cleric clearly best (and only viable option at first level in 1e) with the Paladin & Druid also-rans, to a variety of Priests & the Druid all able the heal well enough, and the Bard & Paladin getting to do a little in 2e, to the Cleric, Druid, Favored Soul, Bard, and Paladin - and anyone able to use a WoCLW, being able to step up in 3e, to the Cleric, Warlord, Bard, Artificer, Shaman, and Ardent all fully contributing leaders in 4e, with the Paladin & others secondary, to Essentials adding a Druid sub-class to that list.Lanefan suggested the solution of recruiting a character to fill the expertise gap. You reiterated that solution with reference to The Hobbit. All I did was point out that such a solution may not be the right one,
5e, while eschewing role, offers fewer traditional-Cleric alternatives than the game had just before it was introduced. The Cleric, Bard & Druid were retained and quite adequate at the role, the Paladin, likewise, stayed a strong secondary, and the Ranger distant one. The Warlord, Shaman, Artificer, and Ardent were all dropped from the PH. The UA Artificer doesn't seem too strongly tending that way, and I don't think we've seen anything quite like an Ardent sub-class for the Mystic. So 5e has a ways to go, to get caught up. And that's just in the one most notorious case.
Really, the only contributions where there's no such issues are DPR and basic (easy-moderate 'warm body') checks.
The same way it flies in an all-caster party.once you give the non-casters said wider range of capacity how will that fly in a party with a more conventional character mix?
Well, not quite the same - the hang-glider will probably have a lower maneuverability class than the Fly spell.

'No' and 'no more so than the casters already are (so kinda, yeah),' respectively.Have you just overpowered the non-casters? Or, have you just made the non-casters into one-man bands where each can do everything and doesn't need anyone else?