No, you’re not wrong about your assumption. When official support for the Samsung device ends, it will no longer receive hardware specific firmware updates. This includes, among other things, the baseband firmware. This also applies to all other smartphone manufacturers. (*). Sustainability isn’t a real issue for manufacturers. They always want to sell only new devices. But this is the same in other branches. There is no such thing as 100% certainty, not even remote certainty.
(*) The life cycle of Wind°ws OS is similar. With the service end of the regular support no more enhancements or improvements of the functionality are provided, but still security patches. The end date for extended support also marks the end of this service (see Wind°ws 7, except for the Windows 7 Extended Security Updates (ESU) for OS from a Volume License Agreement).