Again, every GM in the league...
It is certainly not the case that every GM in the league would have signed JT to a 7 year, $11 mil AAV contract with a full NMC. Even looking at just the AAV (nevermind the player-friendly term and AAV), JT has the 3rd highest cap hit of any centre in the league, behind only McDavid and Matthews. Even when he signed this deal, he was more of a borderline top 10 centre, who was likely to decline over the deal given age/term. Other JT-level centres have not been paid like JT, UFA or not.
Based on rumours at the time, the Isles offered more, but the Leafs offer was 2nd biggest. Assuming that's true, it seems likely the other 29 GMs wouldn't have/didn't given him the offer Dubas did.
Also, team needs are extremely important. The Leafs at the time had 3 really high end, young forwards in Matthews/Marner/Nylander, JT wasn't really a need. Especially since we had a very serviceable 2C in Kadri, on a great contract. Everyone knew our young stars were about to get paid, and that cap space was about to get tight, balling out on a non-team-need was questionable at the time, and turned out to be IMO a very poor decision.
IMO, somebody should have given JT about 7 years, $10 mil AAV, with more of a modified NTC (limited no-trade list) than a full NMC. And that somebody should have been a team with a lot of other pieces in place, that badly needed a 1C. With that slightly better deal, on a team that really needed a 1C, you could look at that and say "we likely made the team better long-term with this signing". However, for us, 7 years, $11 mil AAV, full NMC, given our team needs, it was a bad call. Even if Dubas could have negotiated a modified NTC, we could have shipped JT out already and improved our depth/balance, but that NMC with that huge salary and lack of team need at 1C, it was a major mistake.