Mastodon will probably die first because it's just a software but that's not a problem: ActivityPub, the protocol, and the Fediverse, the network, will most certainly outlast Twitter. Unless Twitter chooses to get compatible with the Fediverse.
A protocol can't die. People are still using IRC, XMPP, good ol' email, decades after they were created. They are still useful, they still work, so there is no reason for them to "die"
