Alright lets see here.
I admittedly dont know much about Thanos (Or Marvel/DC in general), so dont overly angry when i read something wrong or downsell Thanos too much.
But what are the votes actually based on? Thanos only real advantage is his less then 2 times bigger AP and his Skill advantage (Being a 1000 year old warlord and stuff). Marth can migate the AP disatvantage with his Starsphere, not fully obviously but still migate it. Marth can attack Thanos by using his Earthsphere by causing smaller earthquakes, thus migating the skill atvantage of Thanos by a small but existing degree (Because Marth dosnt have to tango with Thanos directly to damage him, thus getting beating in close quarter combat due to skill difference). But the biggest problem i see here is how people undersell Falchions heal here. Marth can actually keep up with Thanos thanks to this. Because now, every blow from Thanos towards Marth isnt long lasting, it gets healed over the course of the fight. Thanos's wounds on the other hand will last for the duel. And now to Marths absolute trump card in this fight: The Dark Sphere. Yes its insanely out of character for Marth to use it. But even Marth will consider using it after seeing his opponent slowly beating him in a drawn out fight (Remember, Marths Sphere's only migate Thanos' advantages, not nullyfies them), Marth is chivalrous, not stupid.
With that glossed over, lets see how the fight would maybe turn out:
Both will engage in close quarter combat, they have no other choice. Marth has a tiny advantage in range through his Falchion, because swords plus arm > arm, even if the arm belongs to a preeetty big man. This dosnt prevent Thanos from flexing with his AP and Skill though, so Marth gets pushed back slowly. His Falchion and the Star and-Lifesphere keep Marth from being pushed back too fast, while the Geosphere plus occasional hits from Marth himself widdles on Thanos stamina. But Marth will still probably lose the exchange simply due the clear advantages on Thanos' side. So now it depends fully on wether or not Marth uses the Darksphere to mess with Thanos's mind to prevent him from harming Marth further, ending the duel in Marth's favor. Or his chivalrous code prevents him from using it and Thanos will win this drawn out exchange.
I already outlined why Marth using the Darksphere is a possibilty in this match so i vote Inconclusive. It fully depends on wether Marth dies first or if he pulles the Darksphere.