When it comes to "better service," speeds are not what people really care about. Most average users give 0 fucks about speeds above 10-15mbps or so. The only way people would notice faster speeds is if they got higher quality video streaming, which they don't because of "video optimization"
The problem with t-mobile's network is all the little holes. All the apartment buildings and office buildings with no service. All the rural towns with no towers for miles. And for these people that can't send a text from their living room, or have to stand in one spot at the end of their driveway to get a call connected (that will still drop 50% of the time), 5G does nothing for these people.
Someone on an iPhone XS or a Galaxy S10 with a poor network experience today, will see no improvement when they are sold an iPhone 13 tomorrow.
The 600mhz frequency is the magic one that can save some people. And on 5G, most of the time, 600mhz still non-standalone and requires a mid-frequency connection as well. So if your on the edge of the network, you want LTE if you want a chance at using your phone.