Manufacturing ASICs from scratch requires a lot of capital, but it is fundamentally possible. There is no way to acquire a permanent, unassailable monopoly over ASIC hardware in general.

It is possible to acquire an unassailable monopoly over PoS tokens. You might be able to buy scraps from random traders, but will the >51% whale be willing to sell their core holdings when they can simply live off their staking yield?

>A permissioned blockchain is one in which the ability to add blocks is limited to a certain collection of entities

I agree. Ripple is an example of a chain which explicitly follows that model. PoS regresses to something like this because a 51% majority attacker can control consensus.