It's the DM's job to make sure the game if fun irrespective of the outcome of a skill roll.
It seems a bit silly to go out for an evening's entertainment and have fun/not have fun hinge on a random dice roll....
The random element is certainly one aspect of fun. Mostly, though, in a cooperative game, the central aspect is contributing to the overall success of the party - it's about as close as you can come to explicitly 'winning' the game, the party succeeds at it's goal (at least, the current stage in it, like, defeating enemies in the current combat, or getting past the current obstacle or whatever).
I think what you're focusing on is another aspect of fun, I'd call it 'engagement.' And, yes, if a given player's character has no relevant/meaningful mechanical contribution to make, it does fall mainly on the DM (though the other players can help) to keep him engaged. It doesn't mean that a class design that leaves the player who chooses it with few or minimal contributions to make in two of the three pillars is "just fine" it just means that the DM & players can have fun, in spite of that deficiency.
The fact that a lot of us have been doing that for decades doesn't mean everyone should always have to do it. (OK, it totally does mean exactly that, but maybe it shouldn't.)