You literally said #2 carrier ahead of AT&T. They're #2 not because of network metrics but because of the amount of subscribers they have. Nationally, they're still ranked last in terms of overall network performance because despite their square milage, they still lack in call and text reliability. There is zero doubt that they have one of, if not the fastest network in the country but that is far from the only metric to measure a network, even if it's the most important to you.
All you have to do is look at the recent reddit thread that suggested T-Mobile might stop their program that gives people those Cellspots to see that a lot of people were ready to switch because the only thing keeping them on was that device giving them service indoors. At least we know Sprint is going all in on the Magic Boxes and they treat it as a part of their network instead of as just another device.
Nothing to do anything about number of customers...
How are they not on par with the big two? TMobile is crushing it for data in all the major markets. If TMobile can execute their 2017 coverage map goal, they will be on par with AT&T, which is considered to be in the same coverage league as VZW. So yes, TMobile will be in this club come 2018.
This is a reach. AT&T and Verizon still have twice as many customers as T-Mobile and even if they added 2 Million customers per quarter it would take 8 years before they got to VZ and AT&T's current size, assuming VZW or AT&T don't have any growth at all.
We can acknowledge T-Mobile's accomplishments without making it seem like they're nearly on par with the big two. In terms of data speed, sure. In terms of price, they're approaching it too. However, you won't be able to find the consistency of the duopoly's network nationwide on T-Mobile right now. That'll take quite a bit more infill sites. That's why given Sprint's "non-announcement" of 8K macro sites for expansion, I'm glad to see it's not a huge expansion because we know that they're going for reliability over square mileage.