How bitcoin maintain its network when bitcoin no longer mineable?
Miners will continue to work and secure the network , instead of getting block rewards they will gain the transaction fees instead.
Who cares really ? None of us will be alive at that point.
what realistic comment
and a bit sad 
But I think someone will survive
with the progress of medicine we can't know our future ... and btc user really young (10 years) probably they can see the last btc mined!