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Fixed supply puts the entire security model at risk, since future transactional demand is unknown. The cap will need to be lifted to subside the reduction of block rewards. Also, POW needs a much higher issuance than POS models to be fend itself from double spend attacks. e.g. ETC has nearly almost been attacked out of existence.

I believe we'll see Cryptos that will run any application including very low cost payments rails with reducing supplies based on extremely high demand (fee burns). Not so different than what BNB is doing or what EIP-1559 has planned.

Moreover, there will most likely be an elastic gold "stable" currency as well where the supply adjust without dilution.

Bitcoin can still exist but it will need to survive a major evolution if it is to thrive imo.