Let's say (hypothetically) governments find a way to shut down Bitcoin or centralize the whole thing. Even if they succeed, Bitcoin will survive because of its open source nature. Meaning, anyone with the code can just re-launch BTC and restore its decentralized capabilities. Not even purging or banning GitHub (or similar) repositories will be able to destroy Bitcoin. And don't get me started on Quantum computing.
At this point, I'd say Bitcoin is immortal. Wouldn't you agree? If not, may I ask why?
Your input would be greatly appreciated. Thank you.

Quantum computers are a real threat and that's why cryptographers already work to combat it, if Bitcoin will not hardfork to combat it as well, it's doomed.
As for re-launching a PoW cryptocurrency, today it's close to impossible to start a legit (no premine, no dev-tax, etc.) project on PoW because it gets attacked pretty quickly, project like this would need to have a very high hashrate from the very beginning, decentralized and ready to combat attacks.
Back in the days when cryptocurrency space was only known to true enthusiasts it was not a problem but today it's a whole different world and you can see plenty of example where a project was destroyed because it had not enough hashrate to secure the network against 51% attacks.