A bitcoin is never lost, Frodo Baggins, a bitcoin exists exactly where it means to
I chose 500 years as never, because I think by then we will have colony in different planets and we will probably shift from the current state of the internet. So, everything that is stuck to this internet might be considered as lost by then.
Even if we inhabit all Solar System planets (something beyond that is quite unrealistic), why do you think the Earth's Internet will suddenly stop existing? Also, Bitcoin does not require Internet at all. Theoretically you could implement the Bitcoin protocol using telegraphs (provided enough consensus, of course).
Quantum entanglement data communication has been demonstrated in the lab and is currently undergoing testing at significant distances.... this should mean, that by the time we are ready to go to other stars, we can have a hugely powerful laser pointed at the destination, that while it will take the speed of light time to first establish the link, will then offer instantaneous communication, so first humans to alpha centauri could possibly have a less laggy internet connection than everyone does now. Provided they remember to fire the laser about 5 years before they get there.... (Not absolutely sure on this, but it may be required to drop off a mirror exactly halfway, easier than freezing the photons for 5 years I assume.)