Looks like that was 2 CEOs ago (Mike Sievert in 2020 and Srini Gopalan in 2025) so one of them probably bears the responsibility for this decline.
2024 might have been the start of forcing us to fight with toasters: https://www.t-mobile.com/news/business/t-mobile-launches-int...
and 2024 seems to also be when they locked down their esim transfer process: https://www.reddit.com/r/tmobile/comments/1cnphk4/android_es...
So I guess Mike Sievert drove t-mobile into the ground?