No question about it.
By far the single biggest reason we haven't moved is that we would experience such a steep decline in salary. The fact that we'd no longer have a mortgage to worry about would soften the blow, but still...
The only cities were you can realistically make a living (at least as a transplant) are Missoula, Great Falls, Billings, and maybe Bozeman. Kalispell has an ever increasing number of rich Californians moving in, which might generate some business. But yes, it would be tough to relocate and start a fresh practice or even join up with an established small firm.
I seem to remember that the starting salary for DA/PD/City Attorney in Los Angeles ($70K) was more than a 20 year veteran would make in Montana.