Not so much as an anti-trust blink these days.
This has direct effect on competition, T-Mobile can raise prices or run it into the ground.
Looks like T-Mobile's found a fix for those "problems".
I've never seen an MVNO do a very successful switch to another network, and if you can't switch networks, you have less negotiating power than consumers.
Apparently it was spun off in 2016, and is now owned by Dish. And looks like they switched to ATT and carrier based calling. I'm just summarizing wikipedia though; I don't know if there were SIM replacements or if the SIMs were network independent so they "just" had to have a different network allow attachment.
I didn't care because I was 99% wifi at the time anyway.