The McCarthy haters won't like it but gimme McCarthy for now. He is what this TEAM needs right now (not just the offense).
We went through this with Jason Garrett 10 years ago. Kellen may be a promising HC one day but I am not ready to let him run an entire team let while he goes through a few years of on-the-job training as HC. There is no guarantee that Kellen would be a huge upgrade over McCarthy as a HC. Do we know if Kellen won't be making the same time management mistakes? Do we know if Kellen can lead an entire team in game prep, practice, etc. during the week? As of now, we just know he's a good playcaller. I believe it just works with McCarthy and Kellen together right now. McCarthy allows Kellen to do what he does best - handle the offense and playcalling while McCarthy does the other stuff a HC has to do.
If some other team wants Kellen then so be it. The guy has some potential but at the end of the day, he still barely has 4 years of coaching experience (sound familiar? Ex: Garrett)
And if Dak is becoming a great/elite QB that people think he is, I would not worry about losing Kellen. The truly elite QBs can pretty much run their own offense once they are established vets.