They are 100% at the mercy of SpaceX accepting their payloads. If SpaceX decides to create an in-house competitor, they are purely and simply screwed. And SpaceX will create an in-house competitor if Impulse Space is profitable.
So they are also betting on either anti-monopoly legislation being set-up (SpaceX is a monopoly because no other launcher can match their pricing), or that a SpaceX competitor emerges (which could easily be a decade out).
It's very similar to the relationship between Apple or Microsoft and developers for their platforms. In those cases if your product makes sense as a component of the OS, or is a table stakes application that the platform vendor can't leave to third parties, then you're in trouble. Outside that, they actually need you to enrich and expand their ecosystem.
Until somebody copies what SpaceX is doing. SpaceX might be first to succeed with this but they are inspiring lots of others to try as well. That's already happening, there are lots of space startups and some are getting some rockets to orbit even. Any patents SpaceX might have (and I think Elon Musk is actually not big into that) would eventually expire and might be licensed in between. I don't think that's a blocker for competition.
If e.g. Boeing wants in on the action, they need to start moving. The main problem with that has more to do with Boeing's inertia than with SpaceX trying to stop them from doing anything. SpaceX has actually been pretty vocal and open about their plans. And by Boeing/NASA standards they are not even spending that much on this. The only thing stopping Boeing from competing here is Boeing being Boeing. Other companies are less encumbered by their own ineptness and will no doubt start figuring things out for themselves.
I'm 95% sure that SpaceX refusing to accept payloads from Impulse Space in order to charge for their own orbital maneuvering vehicles would already be illegal as exclusive dealing.
It still frustrates me that Swarm was basically given a free pass after their illegal satellite deployments[1]. They broke both the law and good cultural norms around space tech in a meaningful way, barely got a slap on the wrist, and then ended up being acquired for quite a good amount of money. The FCC commissioner even stated that "The size of the penalty imposed is probably not significant enough to deter future behavior."
Move fast and break things shouldn't have been acceptable for an organization operating in this area, and that Swarm had no meaningful repercussions for doing so should be a black mark on the org and the executive running it for a long time. I'm certainly hoping others don't use their model as a blueprint.
So they are also betting on either anti-monopoly legislation being set-up (SpaceX is a monopoly because no other launcher can match their pricing), or that a SpaceX competitor emerges (which could easily be a decade out).