Why not 1 billion? Why not 1 trillion?
Keep in mind, at one point the market cap of the Mississippi Company was equivalent of 6.8 trillion dollars (2019 equivalent figure so that would be higher today).
More seriously... although the supply of bitcoins is limited, the supply of alternative cryptocurrencies is not.
IMO the feature set for Bitcoin sucks:
1) There's no meaningful theft protection. If someone gets your keys they have your money. End of story. Unrealistic types might say "I just won't goof up an hand out my keys." Good luck.
2) There's no meaningful extortion protection. Even if you somehow get a computer with no security flaws ever, protecting those keys is a lot easier said than done when someone is mailing your loved one back to you piece by piece. Or when someone's beating you with a $5 wrench until you hand over your crypto wallet.
https://xkcd.com/538/
I can imagine a scenario where this gets so bad, that the major world powers decide to ban all bitcoin exchanges on the charge that they are acting as bagmen for extortionists.
(If they're forced to make the choice between having productive sections of their economy being constantly blackmailed with cyber attacks or wiping out a number of crypto speculators, I can imagine they the might just say "screw the speculators.")
People like to claim that they could never shut down bit coin, and literally that's true. Practically however, the value of bitcoin is tied to a network effect and if you eliminate 99% of endpoints where bitcoin can be exchanged for currency and 90% of user base, the corresponding loss of utility would crash the value to make it virtually worthless.
3) There's almost no privacy. They whole system relies on publicly declaring transactions. It's a gold mine for anyone who wants to do data analysis on you financial transactions.
4) There's no meaningful fraud protection. If someone sells you a faulty product, you can't stop payment.
I think crypto will be around for a long time, but bitcoin is going to go the way of the Apple II and the Commodore 64.