Lanefan
Victoria Rules
This is a double-edged sword, for me. On the one hand I do like the idea of anyone being able to try anything (to a reasonable extent). That said, if the non-specialists' collective odds are good enough to make the specialist redundant, who's ever going to play the specialist?It's not that classes play the same - it's that there's nothing essential that a pc can't try to do. Anyone can attempt to pick locks - but a rogue who specializes in that will do it a lot better. So a party without a rogue isn't stuck when they reach a locked door.
Never mind that the rogue-less party isn't stuck anyway if they have the means to destroy or beat down the door, or spell their way through/past it.
Given as 5e gives you back all your hit points on a long rest, healing of any kind isn't really necessary provided the party rests often.Or, more meaningfully: you don't need magical healing. You have a decent-sized pool of self-healing available. Magical healing is helpful but not essential.
IMO this is a serious bug rather than a feature. As with the lock specialist above, what's the point of having classes if everyone can more or less do everything?
An all-the-same-class party completing an adventure has always been possible, just not always easy.Ergo, a party for four fighters can complete any adventure. The details wpould look different, but it's possible

Having all classes be very good at something while almost completely unable to do other things tends to force a) a mix of classes and archetypes in the party and b) a degree of interdependence between those characters. Which plays right back in to the thread topic: one of the biggest reasons NPCs come in to parties is because they've been recruited to fill holes in the lineup; and I don't mind this at all as it means the party is (for once!) doing something wise, and realistically planning and preparing for a variety of possibilities.