This realistically isn't a major concern because the pool of qualified software engineers in a state like Idaho is far too small to make a noticeable dent in market salary levels.
You're assuming the availability of mobile tech work will not enable more people to move to places with low cost of living, such as Idaho. I believe that it will.
A few people will do that, yes, but there are a couple million software developers in the US, and most of them are not going to suddenly move to sparsely populated interior states. They'll spread out from the SF bay area a bit.