they can be producing and releasing valued content while the rest of the team actually works on the guts.
That is my point, both of them can work at the same time, as WtT and MtG prove.
I don't play Stellaris, so I wouldn't know (pardon my plebeian self, I have my hands plenty full with CK2, EU4 and HoI4, as well as the game I never seem to be able to let go, Vicky2), but in HoI4, with the design of the game revolving around ever sprawling focus trees, content designers always have something to do.
MtG and WtT showed us the most intricate FTs the game has seen yet, and I believe that's precisely because of the extra time they got to work on them, so having content designers and programmers work on mechanics and focuses for big expansions is looking to me to be more beneficial to the game as a whole.
Instead, if content designers work on immersion packs and expansions while programmers work on the expansions, the quality of the FTs might suffer, and as I said, quality always trumps quantity.
And I said a year previously referring to what is to me the holy grail of the modern PDX expansion, Holy Fury, but even just taking WtT into account, it didn't take a year, it was 9 months, and that was just fine.