The mining self-balances; if enough miners shut off their rigs, the difficulty goes down, and it becomes profitable (or break-even) to mine again. (Mining was heavily over-invested, and it's in the ugly process of rebalancing now.) It's possible that Bitcoin will be relegated to a fringe of techno-libertarians, but it is unlikely that the network will ever disappear.

Also, the network is "single-purpose" in the sense that an ethernet cable is single-purpose: while it only accomplishes one thing, it is a building block that can be leveraged in lots of interesting ways (smart contracts, proof-of-extistence, sidechains, colored coins, etc.)