Everyone saying Tanjiro will become a demon - it took Kokushibo 3 days to become a demon. I don't know if he'll be able to metabolize Muzan's blood that quickly. Though, who knows what DotFG breathing can do.
I liked what someone said earlier: that perhaps the concentration of Muzan's cells in Tanjiro will trigger a flashback to the time he was almost slain by Yoriichi. It's well established that those with high blood concentration can see Muzan's memories. In this case, Tanjiro will probably be able to learn the 13th form of DotFG. I find it highly unlikely that Tanjiro will remain human for this fight, though. We have nothing that would be able to instantly change him back. It appears the medicine requires a struggle.
In any case, we still have 2 pillars on the way, as well as 3 of the next gen + Murata. I don't think Zenitsu can use his legs, which is a huge handicap for him. Even if Tanjiro actually died, they should still be able to hold him for an hour, especially when Kanroji and Iguro get their marks & the see through world.