I think the most important thing for its survival is it needs young contributors who care and thus can one day take over, which seems difficult to do consistently.
It will keep evolving until it's faced with a situation where it just can't move on the correct direction. Probably because of some social reason, not a technical one.
It's hard to imagine this happening to Linux in particular because it's ridiculously flexible. But things always change.
In 100 years there will probably be somebody maintaining Linux for sentimental reasons. But it probably won't be in practical use.