I suppose that depends on whether significant enough weaknesses are ever found in the RIPEMD-160, SHA-256, and ECDSA algorithms. If significant enough weaknesses are discovered, then those "lost" bitcoins can be recovered, once again increasing the maximum number of possible millionaires.
However, if such a weakness were found, it could just as well be used to "recover" bitcoins that are currently in other people's wallets and essentially are "lost" to me :-)
This would certainly increase the number of possible bitcoin millionaires, but in practice it would mean that either
- I would become the sole bitcoin multimillionaire (if I knew the secret and didn't tell anybody)
- Everybody could be a bitcoin millionaire for a short moment until the coins are siphoned away again (if everybody knew the trick).
In both cases, owning a million bitcoins would be comparable to owning a million zimbabwean dollars...
Onkel Paul
If such weaknesses are discovered, the cryptographic signature and hash algorithms used by Bitcoin could be updated.
You're right, if weaknesses to all three were "suddenly" discovered by a single person and kept secret, they could steal all the bitcoins. However, realistically, this is not how cryptography becomes broken. It is nearly certain that progressive weaknesses will be discovered slowly over a period of months or, more likely, years. This will allow plenty of time to change the addresses that bitcoins are stored in (excepet the ones that are "lost", which then could eventually be recoverable).
As a practical matter, there can't be anywhere near 20 bitcoin millionaires, since far too many people own portions of bitcoins that they would not give up. But as an mental exercise, the technical details of the bitcoin protocol limit the maximum possible number to 20.
