Not running your own full node implies trust to someone who does, since you lose the ability to validate the chain. And without at least a light node you can not even compute your own wallet balance yourself.
Despite pruning, every node still has to download and keep the data for all currently existing accounts. Not all transactions for all of history, but at the very least every unspent one. Given that everyone on Earth needs an account to participate in this, that would still mean tracking 8 billion accounts on every full node. You save yourself the full chain download (which is good), but you don’t get rid of any of the other problems: the full nodes are going to be DDoSed by their own clients sooner or later.
Pruning also implies trust:
- You lose the ability to verify past transactions (which is one of the advertising points for distributed ledgers)
- if somebody has enough hashing power to compute a substitute block with the same hash and distribute it, or to forge the chain block that includes it, he has effectively re-written the entirety of Bitcoin history.
Regarding scaling to VISA levels, we saw at the peak of bitcoin trading last year that this is simply not the case with the current implementation — and that despite all the optimizations that have taken place in the meantime. Maybe it could be done, but just scaling to VISA levels is pointless — this implies that the bulk of our financial daily lives takes place off the chain, ie in a “trusted” environment of some kind. Which in turn makes me wonder what, then, the use for blockchain is supposed to be in this context.
(I skipped over the fact that bitcoin transactions are horrendously expensive, and bitcoin and friends can’t be used for real-time payment like Visa and most of civilized world’s banking anyway, due to the built-in 10-minute par-block plus waiting for multiple validations lag — since this is an entirely different topic)