While I agree with most of your thoughts, I do not agree with your conclusion. Making software run bad on older computers is bad for sales, as it will let people reconsider the OS they use, and maybe switch (back) to Windows. Rather making it impossible to update seems more likely to promote people to buy new macs as people still see macOS as something that runs fluently ("it just works" was their slogan for a reason) but they would like the new features (split screen in El Capitan for example) that are in the OS, or third party software (or internet content, instead of the browser) begins to run slow. That to me seems like a more healthy strategy. Also, giving your opponents the chance to win the user over with software that runs better than your own software would in my eye negatively promote your OS as well 😉 Though both "solutions" have their benefits and downsides, and I can see how your reasoning works as well, but probably only on people that do not consider switching to Windows an option.