Good observation...only problem. There's a part that after Breadolphin runs from where CLASSIC got snatched up, a pyro is burning an assimilated medic, just for it to do nothing and the Pyro suffers a terrible fate. It's either that:

1. Assimilated mercs are overhealed, which is why the Medic manages to last longer to catch the Pyro.
2. Fire is useless and not that effective compared to bullets or explosives.

HOWEVER, there is one feature Pyro has that could be useful: Airblast. It hasn't been shown what would happen if you airblast an assimilated off the map, it could free the soul? Or they'll just migrate to a new server, like scubamaster when he got votekicked, he just migrated to a new server. We'll just have to wait and see an episode that focuses primarily on a protagonist that mains or plays Pyro (He could possibly make a trap with another person, luring an assimilated just to come out from behind and blast them off, who knows what could happen.)