[shilch]

It's all about the incentives. What is the incentive in IPFS to keep data? If offering it to clients earns you money, what is the incentive to distribute it to competing peers? The blockchain is the solution: A miner must distribute the data in order to claim the block reward (subsidy + fees). Nodes can prune/drop any data they don't care about. You'll end up with specialised and competing nodes where each node only stores a subset of the data - some data might even be completely deleted or only available in archives if it has too little value.