How do you know this? Ever thought that the Avs, Devils and Golden Knights didn't trade their 1st so early BECAUSE it was supposed to be a high pick?
GMs are historically risk-averse. It is why we rarely see prospect for prospect challenge type trades. The closest we've seen in recent years is the Bruins/Leafs Kessel deal, which blew up in Burke's face pick wise because he believed Leafs could challenge for the playoffs. No GM is going to trade a young talent like Nylander for a pick with a highly fluctuating value unless there is a clear threat of an offer sheet. With the volatility of the NHL, a GM isn't going to gamble like that.
It may hold that type of value to the Habs and their fans, but it doesn't league wide. Trading Nylander for a pick that could end up turning on you like the Devils, Avs or Golden Knights pick last year is a significant dent to your job security. It is not firable, but it will be remembered, and give you a significantly shorter leash.