I'm not going to pretend T-Mobile is an angel, but I think they've truly changed the industry.
I don't know about changing the industry, but other carriers have made moves to match T-Mobile, which has resulted in more consumer-friendly options across the board. So kudos to T-Mo for that. But the whole "Un-Carrier" schtick wasn't done from altruism, it was a strategic play decided on when T-Mobile didn't have many options except to be disruptive.
Flash back four years ago and T-Mobile is recognizing the decreasing distance between its rock and its hard place. It was the fourth largest carrier in the US, in a business where scale is EVERYTHING. (Think of it this way: you need 40,000 towers or so to cover the country whether you have 10 million subscribers or 100 million subscribers, so divide up their support costs per customer and...) T-Mo is owned by Deutsche Telekom, which had enjoyed being in the growing US market (compared to Europe) but basically said at this point, "your network is mediocre but making it genuinely good would cost billions and billions of dollars, which we don't want to spend. We will be trying to sell you as soon as we can. Barring that, figure a way out of this and send us a postcard once in a while on how it's going."
Deutsche did in fact shortly agree to sell T-Mobile to AT&T, which ultimately fell through due to FCC/antitrust objections. T-Mo couldn't compete based on economies of scale, and they couldn't compete based on network; their strategy had always been to have good coverage in urban/suburban areas but skip the more rural areas that you need to have really good reach but are not very cost-efficient. T-Mobile basically had to do something creative or die. Given that choice, to their credit, the opted for the former.
With that being said - and even though they have passed Sprint to become the #3 carrier in the US by customers - the fact that they are offering consumer-friendly deals and adding subscribers doesn't mean they are actually in a position to be profitable in the long run. Hint: there's a reason that T-Mobile was engaged in talks to be acquired by Sprint last year, and then again with DISH Network this year... companies with sound long-term economic prospects don't go around seeking to be bought by larger companies.