However Frontier can't legally get rid of them until a replacement carrier is found. That's the whole issue here because the DoT is back at square one trying to find a replaement.

When Great Lakes bid for and was awarded Rhinelander, they intended to start RHI service within a handful of months. And because Frontier could pull out as soon as Great Lakes started, Frontier's days in Rhinelander would be short.

But now Great Lakes has said they can't really guess when they could start Rhinelander, Iron Mountain, Internatinoal Falls and Brainard, and they have asked the DoT to issue new bids to find another airline to take these over. That means Great Lakes won't be taking over Rhinelander, and Frontier will continue to be legally bound to serve Rhinelander indefinitely.

So what the OP said about Great Lakes not being able to start Rhinelander service means Frontier can't discontinue Rhinelander. The DoT will ask for new proposals from carriers interested in RHI, IMT, INL, BRD, and from the time the bids are opened until the time the first flight takes off is often 4-6 months. Personally I wouldn't hesitate to book Frontier out of RHI a few months out because they are legally bound to stay, and they are getting subsidy to do so.

I'm sure Frontier wants out of RHI as soon as they can, but they're pretty much stuck there for the time being.