As always, well thought out and put together post.
I think where we disagree is on the baseline for the core group. To me, keeping it together suggests one more run whereas to you, it seems(correct me if I'm wrong) that adding to it would suggest one more run. This team was really really good, even before they added Coyle and Mojo, The 19 game point streak being the peak of that. They showed that they can do it without Bergeron and Chara and that they have the depth to be very "next man up"
If they weren't in it for one more run, I think Krejci and Krug would probably both be out the door. Don's shown in the past with Lucic and Hamilton (and almost Rask too) that he's not afraid to rip any band aids off when success isn't the #1 focus and he's in asset management mode between windows.
So I look at it like this...standard deviations apply before people jump down my throat
What will stay the same:
Production from Bergeron, Marchand, Krejci, Krug
Goaltending from Tuukka
What will be better:
Coyle with a full camp and year
Pastrnak, DeBrusk, Heinen all probably level up
McAvoy, Gryz, Carlo level up
Clifton to a lesser extent but maybe he surprises
What will be worse:
Chara
Backes
4th line - I expect a slight regression here, especially with the cat being out of the bag from this past postseason.
2nd/3rd line upside options - Last year we started with assumptions about JFK, Donato could be, neither of which worked out. We ended with questions at 2nd, and Mojo on the third. I don't believe we've downgraded much here from where things were on opening night of last year, but no doubt going from Mojo to no Mojo from the playoff run is a downgrade. Even though neither JFK or Donato worked out, they possessed a "if it clicks, it'll be real nice for us" upside that none of the options this year possess unless something Lucic-esque wild at camp happens(Stud, Lauko, Bjork, etc.)
To me, the progress you're going to get, along with the pieces of the core that will stay the same outweighs what will be worse significantly and the B's roll in with a better overall lineup in 2019-20 than they had in 2018-19. No doubt there are still holes, as there were last year, but this team to me is still playing a home game for G1 of the playoffs.
I think the Bruins FO is coming at it from the same angle with the understanding that the window slams the moment the cup is raised in 2020. Otherwise, I don't believe they'd be hanging onto some of the pieces they're currently hanging onto and instead maximizing asset value.
Now this is as of this day. I don't believe the Bruins walk into camp without adding another piece @ wing personally.