a $1mil Bitcoin might be a bit laughable but I don't see any reason why we wouldn't see a $25k bitcoin in the near future. It's hit $20k before so it's not unreasonable to see it rising beyond it's previous high.
And it's all supply and demand. There's only so many bitcoins available, so of course if demand outstrips supply then the value can only rise.
The same goes for most coins of course, unless you're Tether and just keep producing more....