[dboreham]

Fwiw I disagree that the "data availability" concern is real. My take is that anyone with an interest in having some data will go ahead and store that data. If data integrity needs to be preserved under consensus then publish a hash to a blockchain. Possibly we're actually agreeing on this in your use of "essential"?

[DennisP]

By "essential" I mean all the data required to recover complete state.

Holding the data you need yourself might work ok for your own personal ETH balance, assuming you have some way to prove the absence of any outgoing transfers. In fact, that's how one variant of Plasma works.

But with smart contracts, it gets a lot more complicated. It's way simpler if you can just know that any data you need will be on the network.