It's nonsensical to say stuff like this ("that pick could have been [...]"), especially when we're talking a late 1st round pick. Tanguay had a good year with us, and was worth the 1st round pick we paid for him. Never, ever, judge a trade by whom was drafted with said draft pick, because there's no guarantee the team makes the same pick.
Also, that pick was actually used on Nemisz, not Carlson, and it would have been extremely surprising for the Habs to draft a d-man (after drafting McDonagh and Subban the year before, and having a strong need at center), had they kept the pick. So, the pick could have, just as much, actually been Nemisz, Leveillé, Deschamps, McRae, etc. All players who went on to have fantastic NHL careers.