[...]
So it is important that the userbase of Bitcoin must
grow in order for their databases to be flooded with so much data that they can't keep a permanent record of them, but only leave them in their stores for a few days. (see
https://en.wikipedia.org/wiki/XKeyscore#Data_sources)
Have you run the numbers? It would be hilarious, in a way
beating them at their own game by feeding them so much data that they can't process it anymore.
[...]
I am aware of Zerocash, Zerocoin, Zcash and Monero - anything else to have a look at?
Zerocoin and later Zerocash are the protocols which led to the creation of Zcash, not altcoins in their own right. Zcash is not private by default, requires trust in the initial set up, is ran by a centralized entity which awarded themselves 10% of all Zcash ever created, and is transitioning to proof of stake.
I know! Just listing protocols / concepts I've read through in my quest to find something suited for Bitcoin. I guess this could be an interesting topic for a new thread, if it doesn't exist yet.
I have seen nothing which convinces me it is better, or indeed even close to, Monero.
That's interesting. If memory saves correct, Monero is basically CoinJoining all the time, plus hiding amounts. I guess if CoinJoin can be skipped altogether, it would be even better - that's what I like about the aforementioned protocols. But it would be great to discuss in a dedicated thread.