Pruning should arrive in Bitcoin Core 0.11, allowing people to run a full node with only 1 GB of storage space.
Please answer this simple question: "how does one bootstrap a full node once everybody's pruning?".
Suppose every node prunes a 99% of the blockchain, and keeps a random 1% of the blocks.
A new node bootstraps by contacting 100 random nodes.
Exactly, This is how it should be done. The Blockchain should be sharded and sidechains should be used for other purposes like smart contracts or notary databases where not everyone needs to download if they don't need it.
If you have a 'stash' then you can protect it by aligning your own incentives. If you run an unpruned full node then the network can not be compromised and full nodes can still boostrap in a trustless manner. If your 'stash' is sufficient value then you have a very strong incentive to pay the minimal cost of keeping your node unpruned.
Full unpruned nodes will be maintained by decentralist ideologues like myself, payment processors and exchanges, those with a sufficient stake to protect the ecosystem as D+T explained, and other services that provide blockchain explorers or researchers. These sources will always remain sufficient to keep bitcoin decentralized with or without sharding the blockchain.