P2P is a big component of blockchains. The "expensive" part is competition for the limited capacity in global consensus (block space), which is necessary if you want to interact with global systems that deal with high value objects. Blockchains can't exist without P2P, but P2P is not sufficient. There are plenty of use cases that require global consensus, and that part is expensive.
Consensus is only really nessisary for solving problems like payments or namespacing (e.g. namecoin)