If you exclude P2P trades, what else would you do with your unreported wealth in case you legally want to spend it, to buy a house for example?
That's like playing Russian roulette with your capital. If at any time you legitimately try to spend capital that has never been declared, you have some explaining to do about how you got it and why you didn't declare it.
If you haven't used your coins in any way, and that for years, you could say that you miraculously gained back access to your private keys, but you'll be subject to tax from that point.
In all cases you will end up paying tax, unless you just want to get high on your massive on paper wealth that you won't ever be able to spend or use in a legal way. Things aren't easy for people fitting in that category.
It's simple: whoever wants capital gains tax from you must prove there was a gain. Without a paper trail, there's no evidence. Can you think of how to make sure that even if someone who sells e.g. property to you provides Bitcoin transaction information, then there will be no way to prove the money you used was actually taxable? I can.
And there's an even simpler alternative: don't spend your money in jurisdictions where capital gains taxes exist, there are a few.