I don't think he needed to be hitched to them either, but only one coach so far insisted on separating them (but he's an asshole) and they both looked more dangerous apart. Separating them at 5v5 more regularly could also get that 74 pts in 60 games effect for free, right?
I don't know what was said in the coach's office or the dressing room, but I do know Marner's agent publicly aired multiple grievances during the last negotiation, and some of them were: not being allowed to play regular minutes with Matthews, and getting a couple 4th line shifts one game that was super insulting. Once Keefe was onboarded, Marner was immediately paired with Matthews and their playtime goosed too high so there was nothing extra left in the postseason.
For all the magic that pairing created in the regular season, they both tanked with alarming regularity in the playoffs (together). The regular season magic meant the coaching staff was rarely incentivized to try different looks for very long, which left them a bit hamstrung for options when the top line inevitably went cold. Similar with PP1 deployment in the playoffs.
My theory is opposing coaches just game planned them by iso covering Matthews, and Marner kept looking for Matthews, confident that he could pull it off. Some times it worked, and most of the time they got to focus on defense. I think Marner can be effective in the playoffs (we saw glimpses most recently), but it isn't going to work with his default mode of slowing up in the OZ and trying to methodically pick the perfect seam. Multiple coaches may have kept 34-16 together, but multiple opposing coaches snuffed them out rather too easily*.
* I know they put up some numbers occasionally, but they're paid ~11M, and performing at 4-9M levels is a pretty big hit to the depth.