Let's imagine for a second that most people on earth want to hold bitcoin, becuase they believe the price will not go down (i.e., it will remain stable or go up).
Then, they would rather accept bitcoin for payment, were it as easy to accept the fiat of whereever they live. This is because fiat is designed to lose purchasing power.
The way to make it "easy" will be solved by second layer solutions like lightning or even custodial solutions, these need not occur on the bottom layer all the time.
Anyway, to your last sentence. Why it must rise in value.
Satoshi created "number go up" technology which aligns perfectly with human greed and adjusted to occur in large swings on a four-year basis. No growth in price can be linear btw, because how would the front-running look?
Believers in bitcoin believe in "number go up" [1]
[1]: https://www.ministryofnodes.com.au/2021/01/26/what-is-number...