The problem for him is that I doubt Warren retires in 2018 or Markey retires in 2020, so his next chance for advancement would be against Baker in 2018 or the open seat in 2022.
Also, I'm just warning you guys now, once Hillary is either termed out of office or loses, Moulton is becoming my new political obsession. 
Waiting until '22 isn't that bad. It's typical as far as congressional advancement goes.
A slight risk is that he might decide not to run against Baker in '18 leaving an opening for another Democrat who isn't going anywhere until '26.
I could easily see Markey opting out in six years. He'll be in his early 70s, and unlikely to rise high up on seniority.
