[theptip]

That doesn't really get around the problem; I could install a tap upstream of the miner, and interdict any blocks that get mined. It might even be worse than the current model, since the value of the work being done is completely exfiltratable (i.e. if I steal the mined block I get all the reward, whereas I might have to work to extract value from stolen computation results).

[thethirdone]

> if I steal the mined block I get all the reward

How would you steal a block? Changing the target of the payment requires recomputing the nonce.

[theptip]

The value is in the proof of work; just a very hard-to-compute number. All you need to do to steal the work is to prevent the server from sending the hash to the owner, and independently submit that proof of work elsewhere.