If I a company isn't paying dividends or buying back stock, and we (the shareholders) can't coerce said company into doing so at some point, it's essentially an indestructible piggy bank in which money evaporates while you helplessly watch.
One of the big risks that Snap is taking in this IPO is that their common stock has no practical value.
As you said, stock generally has two ways of being worth something: partial control and/or profit sharing.
Snap common stock has neither. It's only value is its ability to be sold to someone else. It's basically a currency? Is $Snap the new BTC? I have no idea.
On a long enough timeline, you have to believe that Snap will either disburse dividends, be acquired by another company or find some other way to convert ownership into actual cash.
I'm not avoiding your question—I agree that it should be based on underlying value—but that seems to be the only reason to buy $SNAP
it's pretty tenuous, but real nonetheless.