I don't think anyone is negating Marner has great value. I wouldn't do dougie straight across for him just based on value alone. But add the fact that CGY has zero need for a player like Marner and this trade makes no sense for the flames. We upgrade our top 6 sure, but gut our top pairing and have no viable replacement for Hamilton.
That's fair from the Flames perspective but from the Leafs perspective- we don't upgrade Marner for anything less than a legitimate top pairing D or a young D with analogous pedigree with some semblance of stability defensively. That is not Hamilton, while he is certainly a clear upgrade over Brodie/Hamonic and their longer periods, of instability, it's simply something we cannot trade a piece like Marner for. The only difference between all of the aforementioned is Gio's value may be underrated which how much he actually shelters everyone else on the back-end.
What this team needs more than anything in the back-end is stability. We'd have more interest in a Tanev than a Hamilton, both have their individual concerns (Tanev's may be more durability related). Hamilton, obviously has more trade value in a vaccuum but it doesn't actually fit a Leaf team need as much as the former.
While Hamilton is better than Gardiner/Brodie, etc. they are all on a similar plane of lethargic, "air-head" plays more often than not. High-risk plays that lead to mistakes are going to happen but at some point, you have to contribute that to a fundamental issue with the player moreso than high-event play. Ultimately, that's not something our young group can hang their hat on right now, which defeats the purpose of any interest we'd have beyond a "buy low option"- similar to how the Flames originally acquired Dougie.