If people lose faith that it will keep going up, then it stops holding value.
It’s pure speculation based on the greater fool theory.
There are a limited number of beanie babies too.
This is being solved, see https://en.wikipedia.org/wiki/Lightning_Network
The increased price makes the system more valuable because it increases the amount of funds that can be transferred through and decreases the cost of that transfer (more order depth for getting into and out of bitcoin).
Today, it's feasible to send $1M with Bitcoin. A few years ago, there wasn't enough market depth to do that very efficiently.
The other 'killer app' is companies/organizations holding all of their funds in a transparent, public way by making their addresses known. I think there will be a lot of value in this, but we're probably a decade away from the space maturing.