If I had my druthers, the Bengals would bring in a QB to compete with Dalton. I do not think Dalton has what it takes to take the Bengals to the next level, especially if you look at the teams that have advanced in the playoffs multiple times over the last 5-10 years. That said, I understand why Marvin doesn't want to do it: 2015 is a now-or-never year for both Marvin and Dalton. If they can't advance past the first round of the playoffs in 2015, I think it's time to blow it up and start over with a new coach, new coordinators, and a new QB. It will be the perfect time - Marvin's contract is up, Dalton no longer has a guarantee, and I don't think either of the current coordinators are anything special - certainly not head coaching material.
Furthermore, I think 2015 is make-or-break for more than just Lewis and Dalton. I think everyone is on notice- with a few notable exceptions. Only the following 8 guys have earned a roster spot beyond 2015 - everyone else must earn their keep and prove their worth next season in order to have a spot in 2016:
RB Gio Bernard
RB Jeremy Hill
WR AJ Green
FB/TE Ryan Hewitt
OG Kevin Zeitler
OG Clint Boling (assuming he's re-signed)
DE Carlos Dunlap
LB Vontaze Burfict
CB Dre Kirkpatrick
CB Darqueze Dennard
FS Reggie Nelson
SS George Iloka
Not on this list:
Geno Atkins - must improve on 2014 performance in order to justify his salary in 2016
Pacman Jones - could still be around as he was good this year but is getting long in the tooth
Leon Hall - looks washed up. Barring a bounceback 2015, he could be finished
Tyler Eifert - needs to prove he can stay healthy and produce in 2015. If he does, he should be a longterm fixture.
Russell Bodine - jury is still out although he was decent for a rookie. I'd expect him to be a longterm starter but want to see a little more in 2015.
Mo Sanu - proved in the second half that he is not a #2 WR. Hopefully the team realizes that too.
Marvin Jones - needs to show he can shake off 2014's injuries to return to 2013-level productivity.
Andrew Whitworth - not a given he'll still be playing after next year.
Andre Smith - likely gone via FA after 2015
All other DL and LB - need to prove their value in 2015, 2014 was disappointing for all except maybe Malauga. Still, Peko, and Hunt were particularly disappointing in 2014 and I wouldn't expect any of them to be around after next year.
Again, not saying all or even any of the guys listed above should be pushed out of town, just that they need to produce in 2015 in order to earn their spots as part of this team's future in 2016 and beyond. Nothing is guaranteed, especially with them.