DW is, for better and worse, irreplaceable. Fox would need to go in a different direction with their booth. Another factor is that there have been a lot of rumblings about how long Gordon wants to do it, with a more active role at HMS awaiting him.
If Gordon had arrived post-DW, maybe he would feel like the star of the show, as Dale Jr. does on NBC. As it happened, he was sliding into an established team and has been quite laid back out of respect for Waltrip and Joy.
It will be interesting to see if this turns into Fox blowing up the booth and starting over, or whether they try to manage gradual transitions. In five years, it's probably a safe bet that none of the current team will be there.
While Fox already employs a lot of on-air talent, I'm not sure any of them have the star power to step in. If this had happened two years ago, I would be surprised if Fox didn't make a strong run at Carl Edwards. Now, he's so far off the radar, who knows. I'll tell you who would be fun and solid, but it won't happen because he's too polarizing: Kurt.