It's up to the network operator to efficiently manage their spectrum in a way that does not interfere with other radio users, and mutually between their own base stations.
If T-Mobile/Starlink manage to do that, you get additional connectivity in remote areas on your existing devices. If they don't, and their own network quality degrades, you can switch networks; if they degrade other networks' quality (actually, not just as a hypothetical), hopefully the FCC will shut them down.
That isn't how antennas work. Cell phones are pretty low-power devices, but a station in space is going to be much much more powerful than cell phones. Any tower with an antenna tuned to this frequency is going to have to dump that power somewhere. This is called "front-end overload" and is something most things should be protected against, but there are probably many devices that don't expect several watts of power being dumped on them from a single direction.
The latest generation of satellites use software-driven phase array beam forming antennas, effectively behaving like it has hundreds or thousands of tiny spot beams. That allows it "talk" on the same frequency to different receivers much closer (geographically) to each other than traditional systems.
Think beam-steering MIMO WiFi where a base station can direct the vast majority of its energy directly toward a client, meaning clients in other directions never see the transmission and so the noise floor is lower and bandwidth higher. Similarly such systems can "see" which direction a transmission comes from and so distinguish multiple clients transmitting simultaneously - much the same way your eyes have no trouble seeing two people waving at you from different directions even if they're twins and dressed identically: the spatial information is sufficient to tell them apart.
To a rough approximation the delta between the theoretical limit and practical limit is computationally driven these days and we've gotten really good at the computation part.
What kind of power supply are you imagining these Starlink cell satellites to have? You'll absolutely not see "several watts of power being dumped" on anyone, even assuming extremely small spot beam sizes.
This is an extremely power limited application. We're talking signals just above the noise floor here.
It's up to the network operator to efficiently manage their spectrum in a way that does not interfere with other radio users, and mutually between their own base stations.
If T-Mobile/Starlink manage to do that, you get additional connectivity in remote areas on your existing devices. If they don't, and their own network quality degrades, you can switch networks; if they degrade other networks' quality (actually, not just as a hypothetical), hopefully the FCC will shut them down.