How do we know about the protectability of an "idea"?

There are many ways a business can create a defensible moat (as Warren Buffet puts it). It can be the fanaticism of the fanbase (Bob Dylan etc), could be network effects, could be regulatory burdens on competitors that you are exempt from (Indian reservations) or even a unique location (NZ no software patents). But note that most IP provisions are intended to be used as shield (against rip-off, passing off, leapfrog of sunk costs) and not sword. Only patents have that position and software patents are not trivial to gain. There are tactical plays you can try depending on the business (Indian rope trick), or the stairwell method that Telsa uses in continually dropping prices (economies of scale) so nobody except well-financed companies can enter. But I'd just note that most network B2C activities have pretty much been occupied (see Union Square investment thesis 2.0) so you have to work a bit harder like unique proprietary tech or ultra-loyal DevOps willing to go extra mile.