You really shouldn't speak of things you clearly know nothing about.
MVNOs pay the carriers wholesale rates for access to their network. The service can be deprioritized (as my AT&T plan is) or can be at the same priority as one of the carrier's own customers (at an additional cost).
My AT&T plan isn't congested on LTE *OR* 5G so again, you don't know what you're talking about, as being deprioritized (I intentionally chose a deprioritized plan as it was cheaper and I had full knowledge of the capabilites of AT&T's network here) would mean if the network was congested, I would have poor performance.
Your issue with T-Mobile was one of congestion. Plain and simple. I'm sorry that you don't understand how networks work or that you have drank the Kool-Aid of T-Mobile's advertising but if an LTE network lacks congestion in a given area, there is no need for 5G. It is absolutely possible to build a network, like AT&T has here, that has sufficient capacity without 5G.
Also, please note, I never said 5G has no benefits, I said that on AT&T specifically, there is no point to having 5G without C-band availability. T-Mobile and AT&T are not comparable here as T-Mobile had few restrictions put on their midband while AT&T and Verizon have had to contend with everything from the DoD still using the airwaves to satellite interference (which is why Hawaii will never have C-band) to the airlines throwing a fit because they're too cheap to update their altimeters and want the carriers to foot the bill. Until AT&T gets C-band, the 5G icon in most areas is nothing more than 5MHz of band n5 spectrum, not something that's going to make or break LTE that already works fine.