However, you'll have evaded wealth tax for years in case you lived in a country collecting such a tax.
If it's several million dollars worth you are talking about, then that is more than enough money to easily migrate to a country with more friendly bitcoin laws if you are concerned about a wealth tax.
I wonder what has happened in scenarios where people have lost access to their wallets or keys for a number of years (and therefore not paid wealth tax since they don't technically own that money) before managing to recover them, find an old back up, brute force them with newer technology, etc. Have tax authorities gone after them for historical wealth taxes?
That's an interesting scenario. If we try to translate it to a pre-crypto scenario, it would be something like losing a bag of gold coins, then finding another equal sized one in your garden years later for example. It should be handled the same way with lost wallet / wallet password. It's hard to prove that you don't have a backup somewhere, though. I would argue it's impossible to prove there is no backup in some drawer. So in the end it would depend on your law: if it's your burden to prove you don't own something or if it's the authorities' burden to prove you do. I'm not a lawyer and have no idea how this is handled in any country, but it would be interesting to know.