Every class is viable in 6v6. They're just viable in different situations. Scout, Soldier, Demoman, and Medic are the "standard" loadout because they are either A) versatile, all-around fighters (Scout, Soldier) or B) incredibly useful at all times (Medic, Demo). The reasons classes like Heavy, Engineer, Spy, etc aren't run 24/7 because they are the niche classes--the polar opposite of the good all-around fighters like Scout and Soldier. Instead of the #yolo plays, we need to focus on whether or not Pyro is viable to run full time. The answer, as of now, is no. Pyro has only close-range viability and can get destroyed by a Scout. It's designed solely as a flanking class, and, without the speed of a Scout or the mobility of a Soldier, Pyro just gets beat long-term by having a Roamer or a Scout. The airblast has been explored in and out in the meta and the only ~reliable~ use it has is to airblast back ubers, and with the advent of the QF even that is being somewhat neglected.
Overall: Is Pyro Viable as a full-time class in 6v6? No.