Both times they won the Stanley cup they had Kucherov (5"11),Point (5"10) and Palat (6"0) with the rest of the top 6 being 6"1+
In the hypothetical scenario where this really happened and Hughes backed out of the trade, I could see the reasoning behind it. That would've made quite a soft top 6 with Caufield, Suzuki, Zegras and Demidov.
We could also argue that despite Mailloux reputation, he's still a very valuable piece. You don't get too many 6"3 RHD with a mean streak that can skate, move the puck and put points on the board. He's going to be a legit top 4 D for a long time. Apparently, they also really, really liked Hage, so that could have weighed in their decision to pass on the trade.
Would I have made the trade? Probably not after drafting Demidov either. I feel like he's going to play a similar role to Zegras would have played in Montreal and Mailloux is now an important piece of our future since Barron seems to have stagnated. I'm not sure we can afford to trade him at this point. However, if Montreal missed out on Demidov and drafted a D instead, it's a trade I would have done for sure.