Casteel was already offered the Michigan DC job and passed on it. Why would he leave now?
Bill Stewart may be on the "hot seat", but he has not been fired. WVU is 5-3 with all the losses coming by 1 score, including @LSU. It is very premature to assume he's gone.
Even if Stewart is fired, the most likely replacement would be... Casteel. He has roots in the area and is a very successful coordinator. Why would WVU rid itself of the best asset they have? Casteel has been entrenched as a coordinator for years and might feel comfortable promoting one of his positional coaches to DC and moving up.
Consider the alternative. WVU isn't that attractive to many head coaching candidates. While some ambitious up-and-comer might take the job as a stepping stone, that scenario brings its own problem. WVU seems to have a unique sort of culture that not every outsider is going to "get". Hiring from within is probably their best option.
I'm no expert on WVU football, so let me know if the above depiction is wrong. Casteel seems like wishful thinking to me (assuming you like the guy, which I realize many do not.)