I agree with most of what you say here (Heiskanen is the best asset in the deal, Toronto would have to add to Marner) however, this isn't some homer thing where "most Leafs fans" believe. It is a simple fact that Sandin is worth more than a late round first (and won't be added to any deal).
Late first round picks hit on occasion and completely miss on occasion. The Leafs' look like they got a strong hit here and the chances of the Leafs landing a player with a late first round pick that is close to Sandin's value is under 10%. Maybe he doesn't pan out, but his value now is still his value now. I looked at all the late first round D drafted from 2005 until 2018. There were 43 D drafted late (pick 21 onward) in the first round over that 14 year period. It included John Carlson, Niskanen, Maatta on the one end and plenty who never played a game in the NHL or played single digits on the other end. In fact the median career NHL games played for those 43 D is 42 games. Only 5 of the 43 played NHL games before turning 20 (Maatta 78 games, Jokiharju 38, Sandin 27, Jacob Larsson 4, John Carlson 3). Only Sandin was best D at the WJC, although Carlson was also on the all-star team. Only Shea Theodore (87 pts in 108 GP) had better production in the AHL, with John Carlson (48 pts in 77 GP) having roughly the same as Sandin (53 pts in 78 GP and Sandin was younger than either). Other GMs want Sandin because they know his value is way higher than a late first round, but the Leafs' wont be trading him.