Why memo (the protocol) is important is not due to storage but due to uncensorable communication and identity ownership. All OP_RETURN data can be pruned.
It's uncensorable because it is stored on the blockchain. If IPFS suffices for storage then why would you need to use the blockchain at all?
Because there isn't ownership in IPFS. The blockchain allows identity ownership. Also, all transactions are viewed the same as long as you pay, so communications (txs) aren't censored
