1. It may sound harsh but your solution seems pointless for now.
2. Your 'ultimate storage' grows with more users, but so does the amount of spam produced. It would solve nothing. I like many others would still prefer to store the entire chain.
3. Current fix = limited/no spam.
4. Pruning, would remove all spent transactions that are 2(?) transactions back since they wouldnt be needed, dramatically reducing the size of the blockchain. At which point your solution is entirely moot since anyone could store whats left of it without issue.
5. Storage devices are getting cheaper and larger every day and so is memory. im sure if it were needed at some point in the future someone could build a custom board with a crazy amount of memory on it to store the UTXO set. With the speed memory runs at im sure someone could make a slower, cheaper, larger ramdisks for this purpose.