If people want to play pyro in pugs then they're going to have to be willing to play him in leagues first. Scrims and matches are the best way for people to learn how to counter and play against certain playstyles. Once people start getting the hang of countering pyro, then I feel that it would be ok for him to be allowed full time in pugs. Pugs are too unorganized for pyro to be allowed fully right off the bat, because of how powerful the class is in a semi-chaotic environment.
This needs two things:
The first is that there must be more teams willing to run a pyro full time over the course of a season, and Olgha's team and a team in TFCL are the only two I can think of.
The second is that people must express the opinion that pyro is fine to face after facing these teams, which is the hardest hurdle to get over, as that class is still annoying as fuck to face even when you're beating it.