The internet like several large systems people depend on were created by major institutions for major institutions and any attack on them would weaken the institution doing the attacking. Bitcoin however, was created mainly to circumvent major institutions, so they have no vested interest in protecting it unless there exists 3rd party grassroots pressure to do so.

Big difference is that Bitcoin has all the right conditions for a bubble pop. If someone did a 51% attack, this would cause people to lose trust in the currency, everyone would cash out, creating a bank run, which would be a self-accelerating process that drives the price even lower. So even if the network recovered the next day, it would take a long long time to rebuild that trust.

Whereas with the internet, if someone is able to knock out critical infrastructure one day, engineers would figure out a way to fix/reroute it, get it working a few days later, and everyone would happily log back on as soon as it's working. Although maybe they would throw out their "smart" coffee machines and etc.

I disagree that everyone would cash out.

There have been crashes of bitcoin before. None "to zero" but pretty substantial, and not everyone "cashes out" right away.

Sure you're right, some people would stay invested, some people would even re-invest during the price drop (dead cat bounce).

But the previous crashes weren't caused by any real news of a significant weakness of Bitcoin, so if there was news like that, then the crash would be worse than we've seen.

Sure, but the internet serves the economic purposes of most of the world. Nobody wants it to go away. Can't say the same about bitcoin however.

And that sentiment towards Bitcoin is changing every single day.