I also own a 2019 MBP 16-inch, and use Windows 10 via Parallels Desktop. Why would you need to upgrade the OS, aside from security updates? There’s nothing forcing you to do so, and there is no “ticking clock” that suddenly appeared because Apple stopped releasing Intel models.
Just because Apple has phased out Intel chips doesn’t mean that they won’t support what you are using through the warranty and extended warranty (AppleCare) period. Apple computers generally have seven years of hardware availability before being classified as “obsolete” by Apple, so even if something goes wrong in the next three years with the hardware, you should be fine.
Let's say Apple decides that the next operating system after Sonoma doesn't support Intel. So I stick with Sonoma - in principle no problem, as you say, and Apple will keep producing security updates for that OS long after my laptop becomes so old in the tooth I no longer want to use it. So far, so good.
But one of my major Windows workloads is Visual Studio, and developing apps for both iOS and Android using the cross platform tools that it provides. I work with Windows hosted in VMware. This setup is really useful, because my iOS build host can be my host laptop, accessed from within the guest Windows OS by the build tools. Everything running on one machine. Highly convenient, cost effective, performant and mobile. The problem is that I know from past experience that the cross platform development tools tend to work best when the build host is running the latest OS.
So if I end up stuck on Sonoma on my Intel host, and the tools running in my guest move forward with Apple's OS releases, I'll likely eventually end up with problematic build cycles. So that's one reason for me to want to keep my MacBook up to date.
Also, being fully in the Apple ecosystem, I'll generally want to keep all my devices up to date if possible, to get the best interoperation.
There's nothing actually forcing me to upgrade OS. But generally things will work out better if I do, eventually.
