I think échanges (middlemen between miners and users) were a temporary distribution system in the early days of bitcoin and will fall by the waste side as the supply becomes more widely distributed.

Custodians or bitcoin banks will always exist, as long as they offer a better service then self custody and don't go to crazy with fractional reserve since history has shown with a digital asset that can be completely withdrawn overnight you need to keep majority of deposits on hand, in kind, ready to withdraw.