The upstarts can try to undercut the big guys, but they still are stuck at a 20% profit margin; if they make too much, they have to refund the excess premiums.
It's quite clear that the insurers are aggressively trying to control costs when you consider that networks on exchanges are extremely narrow and denial rates are actually pretty high - IIRC nearly 1/3 of claims are initially denied for ACA plans.
These companies are also bringing the narrow network model to the small and large group markets too, where utilization review is also quite aggressive and has been for a long time.
It basically works out like textbook oligopolistic competition models where the Nash equilibrium resembles a prisoner's dilemma. For companies to increase "total market size" by allowing costs to balloon would require unusual collusion and there's no evidence of it.
Narrow networks are a big problem, having been denied access myself while trying to be in network.
Narrow networks are usually a symptom of few and picky doctors imo. But I don't really know. Economics suggests that more insurers in a market should benefit providers.
I and others have been denied enrollment because the "panel is full". Narrow networks are not always because the physician doesn't want in. Sometimes it's a deliberate insurance strategy.
Btw, in California the state medical and hospital associations are definitely more influential than insurers. And I bet that's true in MA too.