There are lots of products and services that require massive amounts of resources, time, and research in order to compete. Like spacecraft, military aircraft, pharmaceuticals, etc. Basically you can only compete on those fronts if you are a billionare/huge corporation. It would be very hard for a new player to compete with SpaceX given their head start. That doesn't mean SpaceX has a monopoly on spacecraft, you can always buy a Russian rocket.

A monopoly (in my mind) would be more like Comcast. Since they've lobbied for laws in a lot of places that make it extremely hard to dig up the ground and put in new telecomm infrastructure, they completely control the supply of terrestrial internet service and there are literally no other ISP choices in many places. Thus, your only alternatives are non-terrestrial ISPs (LTE, satellite, etc).