Blashill isn't going to allow Mantha his growing pains if it's going to hurt the team. This is the same situation with Tatar/Nyquist and Cleary two years ago, for example. At the time, Cleary actually WAS better than Tatar and Nyquist in a vacuum. But everyone could also see that with decent linemates and minutes at the NHL level, Tatar/Nyquist would quickly leapfrog Cleary (or Bert. Or Sammy). The thing is, was Babs going to allow less effective shifts on a nightly basis for the sake of expediting Tatar/Nyquist's development? Nope.
This is the same thing. People disgruntled with Mantha's treatment are forgetting that Blashill is trying out for an NHL job right now. If Babs leaves, he wants to make his case for being the successor. If Babs stays, Holland will let him have his talks with other teams. This guy wants to win games and series. His primary goal is not development. Is he going to sacrifice a stretch of games by putting Mantha on the top line for 20 minutes with Miele? Well it could potentially do wonders for Mantha's development but it could also cost him points while Mantha adapts. He just isn't going to do that right now.
Unfortunately, Mantha is going to have to make his case for promotion with questionable linemates and little time. We all know how difficult that is for guys who have faced adversity. It's undoubtedly tougher for a guy who has been his team's go-to guy a few years.