Honestly, and no offence meant but I think you (and many others) must just give up this idea of somehow keeping old macOS's running to support older software if it is on your primary device. If you have multiple devices, sure, running a 2009 mac with Snow Leopard on board to run old apps and games from that era is fine but it's an impossible ask if, like me, you only have one device.
I only have a 2015 MBP and keeping it up to date is essential for security, syncing and compatibility with other devices and software. Change is inevitable and so is upgrading. When my MBP is unable to upgrade I will buy a new one.
You can stay on one macOS for three years before Apple makes it unsupported which should be plenty of time to find new software and learn about predicted changes. A lot of people work one macOS behind which is a good idea in many ways and gives time to absorb the implications of upgrading as well as ensuring all the bugs have been ironed out but if you buy a new iPhone or iPad or simply upgrade their operating systems you may run into compatibility problems again.
Of course Apple and other developers make money out of this constant change but thats how the system has always worked, they do not change just for the sake of change, its about competition, profit and progress. If not for those motivators we would still be using D cell batteries and incandescent light globes.