Sure but ipfs hosting etc rely on one or a few hosts; if you use a blockchain like btc, at least it is vast and that means someone will be hosting at least until btc reaches $0.

not necessarily. BTC has UTXO (unspent transaction output), whereas ETH has proper accounts and the money is a property of the account.

so if any of these chains only want to focus on sending/storing money (or simply address the evergrowing size of the chain), then they can start purging the old transactions from the ledger.

(yes, ETH has smart contracts, but it's not impossible to imagine that ETH will start charging gas for upkeep of that data, and when the smart contract runs out of upkeep it gets deleted.)

Yes, but most other way of storage are even easier to purge. Also these people are not exactly sane, so I can see them spend millions on ‘securing the documents on chain’.