Does bitcoin price eventually become stagnant?

When all of the 21million bitcoin is owned by the true HODLers as an investment asset rather than speculative currency and it's not being traded back for fiat anymore, how its value is going to continue increasing? Essentially, if there's no more bitcoin for sale in exchange for fiat, is that exchange rate going to stay constant? Or is the base assumption that there will always be someone to budge and sell at the right price for them? Why wouldn't they use their bitcoin holdings as a collateral for a fiat loan instead of selling it?