You have always had the choice to buy your own modem. You were actually paying them for the benefit of using their modem, so ball is in their court. You could have also turned off the xfinitywifi SSID on their modem, but I’ve seen it reactivate itself.

What you experienced in your apartment is more related to how cable internet works. It can be bogged down by a lot of people nearby using their own cable internet. I lived in a townhome complex in college that this happened at. The difference was unreal between summer and fall. Whether copper cable, or fibre, they all eventually trunk into a smaller number of cables that are effectively shared by many people. Fibre is less susceptible to a noticeable bottleneck because of how much more bandwidth it has over copper wires.