From what I've seen, the main purpose of like 90+% of altcoins is to benefit their developers and investors and they'll likely all die and disappear.

They shouldn't, what Bitcoin is enabling is the ability for whoever to use whatever kind of coin they want. Bitcoin opened the whole pandoras box where anyone can create their own coin if they want.

However, for each crypto coin there is clear incentive to make the coin more useful in transactions, both for those more well off and for those less fortunate. Typical BTC holder wants to make BTC useful for everyone, because that adds value for their holdings. That usefulness might add value to those less well-off. If not, they can use something else - in the end there is more choice in the marketplace which should (in theory) benefit everyone.

Just taking a stab at it, maybe bc we haven't come up with a notion of how a viable crypto-coin could be started by the least well off, who are -by definition- at a competitive disadvantage in launching successful projects.

I'm not saying it's an impossible problem, just that these systems tend to be driven by economies of scale, whether it's new/disruptive, or the product of an older, more established regime.

And to be clear, I am not suggesting this with the finality of a foregone conclusion, but merely thinking aloud about the dynamic of the underlying problem.