As long as the internet exists, I will agree with you, but if we wake up one day and anything that has to do with the internet is shut down, then bitcoin will go down with it. There is no way to communicate with the technology without an internet connection.
Aside from that, Bitcoin being decentralised alone has defeated the challenge of it being controlled and shut down by any centralised entity.
If the government decides to ban Bitcoin and make it criminal to own it. People and institutions will avoid using it for fear of punishment. If there are no Bitcoin exchanges because the government has shut them down, Bitcoin will lose its liquidity which will discourage investors. Bitcoin is indeed immortal but government policies can make it lose its worth.
People are so focused on the decentralization of bitcoin that they become complacent and think that even the government cannot stop it. That may be true but people forget that the government doesn't need to find a way to shut it down. They just need to ban it and declare bitcoin illegal, bitcoin will gradually become worthless and bitcoin's immortality will no longer have any meaning.
When bitcoin becomes worthless, will we still support bitcoin? I don't think anyone is here just to support and maintain the existence of bitcoin without caring about their own interests. Bitcoin's existence is not determined by its decentralization but by human need, if we no longer need it and do not want it anymore, it will die.