I think it has been a mini market inefficiency and teams will continue to look for it but it might become too popular, like, immediately.
Not sure I would put King in that bucket. Wasn't his more of a long term rehab thing?
It's a thing that has always happened at some level. It's more about identifying arms that could do it than jumping on a trend. It will probably always be a trend at some level because our understanding of what makes a good pitcher, and therefore a good SP, changes over time and some teams have differing opinions.
The Rays do it all the time. E.g., Zack Littell was not starting when TBR acquired him and he became a full time, good SP. When they trade for guys like Bryan Baker I always half expect that to be a SP candidate the next year. Ostensibly Baker has a decent FB, two decent offspeed pitches, and good command. Why not?
CJ Wilson was a huge success conversion 15 years ago. Texas then tried to do it with Neftali Feliz and it ruined his career. A couple of years before that, Ryan Dempster successfully went back into the rotation after 4 years as a RP. Derek Lowe did it even earlier, at 29 years old, with great success.
Shaun Marcum was a full time closer in College and was abruptly converted into a SP in the minors.