A new manager has to come in and immediately assess whether they have a chance to make this team into anything over the next couple of seasons. And they have to do so honestly...not simply come in and say it can be turned around in a hurry if it can't.
If it can't then you do need to seriously look at moving Horvat and Miller given they are UFA in two years time. And yes I would look to make those moves right away if that was the decision. It would hurt to move Horvat but it could also give an actual rebuild the kick it needs. Provided there was competence in the office making that move.
The people who love Jimmy Potatoes love to say "yeah but this team is close" but they don't acknowledge that there's only one player with long term security in the organization - Demko. Our young players need big contracts and will need a follow up long term deal within the next 2-3 years. Like you said, Bo and Miller expire in two years.
They need a top pairing level RHD, a second pair D to partner with Schmidt, and an elite 3rd line center.
They also do not have the assets to dump their bad deals or acquire the pieces. Their only expandable assets are total shit. I think the only truly expandable young assets are one of Hughes/Rathbone due to redundancy and this upcoming 1st (even though the cupboards are already bare).
To get this, they need to dump the salaries of guys like Eriksson, Roussel, Beagle, Holtby, Virtanen, and Myers. Myers cannot stay. He is not a fit with any of Rathbone, Schmidt, or Hughes. This is $25MM of capspace.
If you don't think there is an avenue towards getting those three pieces within the next season and Horvat/Miller's asks are excessive, the immediate window is dead.
So no, this isn't an easy situation with all the core pieces in place considering this isn't Toronto and not all the long term pieces are locked up and that a majority of the season, we iced 5 blue liners over the age of 30 with the one under 30 being one of the worst defensive players in the NHL.