You don't need a full copy of the historical chain to create the UTXO
I think you have that backwards. How can you create the UTXO if you don't have a full copy of the chain?
a DHT can ensure that a full copy exists even if you only have a portion of it.
I don't think it can ensure that. All the nodes storing a certain block could disappear from the network at the same time. Where's the full copy now?
i like the proposal that any node keeps some configurable amount of transactions (say 10% to be VERY conservative)
edit: afaik it should be enough if the hole blockchain is anywhere (bitcoin foundation?) to download from. with -rescan it should do the same? please correct me...