No offense to US Mobile, but that not a real comparison. No one is paying $120/month & getting the same thing you are getting at US Mobile. The highest plan that T Mobile offers is $100/month, but that includes an upgraded phone ever year, Satellite connectivity, Netflix & Hulu included, $10 discount on Apple TV, 4k UHD streaming, unlimited hotspot, unlimited premium data, and texts & high-speed data access in 215 countries.
The price delta is a lot closer than that for most people. My T Mobile plan includes 5 phone lines, an ipad, a watch & backup home internet for $210/month. Excluding the first year promo, it looks like US Mobile is $32.50/phone line (if you pay for the full year up front), so I'd be at $162.50 for just the phones & watch Since that is free with the $32.50 plan on US Mobile. (If you don't pay in full up front, the USMobile plan for just 5 phones would actually cost me $10 more per month than I am currently paying T Mobile). But I still have to add in a tablet data plan & backup home internet. Neither of which US mobile offers (that I can see from their website). They do have a tie-up with Starlink, but you can't access the pricing without creating an account. The best priced Starlink plan that was available when I looked at this earlier in the year was $10/month but included no data.
So, for $47.50 more per month, I get home internet, and an iPad data plan. Plus T Mobile Tuesdays, and free hulu and netflix. And higher priority calling if/when the network gets congested. and the ability to not have to spend $2K on cell service up front, at the beginning of the year. Is that worth $47.50? For some people yes, for others not, but it's a lot closer than it looks at first glance.
And with Verizon's new $30/month plan, I'll bet it's even closer than that (but I haven't looked at the details). I'm not bashing USMobile, there seem to be a lot of happy customers in this chat, and without the MVNO's, I'm certain the Big 3 customers would be paying more than we currently are. But it's not quite a simple of a decision as painted out to be above.