Many but not all. Sia for instance will continue to exist independent of the company, it is fully decentralised. I believe your statement is true of StorJ as they rely on centralised services but I can't comment on the others.

Bitcoin would continue to exist without the exchanges and wallets, they are peripheral to the core technology. Blockchains can't be 'forked at a whim' without majority consensus, that's the whole point. ETH is the obvious example where the devs forked with support from the community and it led to the ETH/ETC divide.