No he didn't. Brown and Desrochers played better at BOTH ends. Listen Mermis could end up being decent in the future but to say he was the best D even on his own team is a stretch.
Brown and Desrochers did get better as the year went on but much of that can be attributed to Mermis coming in and solidifying the generals as a whole.
Desrochers was playing third line minutes against easier competition after the trade. Desrochers is decent but I think you are overating him quite a bit. We'll see how good he actually is once he plays any sort of quality competition. Also, he got burned repeatedly when Oshawa had last change by guys like Paul and McDavid so I would be careful when I say that he is better at both ends than the go to shutdown guy on the team. Also, he finished with less points than Mermis despite getting more power play opportunities.
Brown and Mermis had instant chemistry. Mermis' speed makes up for Brown's lack of speed and puck moving ability. Brown's size makes up for Mermis' slightly below average size. Thing is that Brown is a little slugish to me. He has a hard time moving the puck out with a breakout pass and instead relies on chipping the puck off the boards. Does it work? yeah but it does not create much offensively the majority of the time.
This is just my opinion on the matter. If I were a GM, I would certainly invite Mermis to a tryout and see how he does. I watched the gens all year and they became much more defensively sound during the second half of the season. I believe he is part of the reason why this happened. Does he become an NHL player? maybe not but I can guarantee that he becomes a good AHL player and Saint Johns is a little low on those at the moment. Also he is LD which is a priority for the habs AHL club