bitcoin price will be around 4000-5000 USD per coin one day and that is already a couple of 100% rises and it will only happen slowly. i think something around 100% rise each year (give or take) is a good estimate of the future rises.
The thing with that is
BTCBTCBTC will lose it's purpose, at least when when it comes to doing microtransactions.
No it will not. Bitcoin will never lose its purpose as long as the dark markets are around. If you can give a good alternative to Bitcoin to be used by drug vendors, internet criminals and scammers, ransomware hackers and other underground markets that are not served by the banks then that it might lose its purpose.
I think the ZCash is more suitable for the dark market.