2 more years and your mac will not install the latest MacOS version, which then means you will no longer be able to update any software on your own computer or install any new software either. If you could install linux or any other OS on it you would be able to use it for decades to come, but instead you will be forced to buy another new shiny Apple box.

You will probably argue thats ok because you would have wanted one anyway by that time or some such point, but the fact remains that Apple will force you to upgrade your hardware not very long away by not allowing you to install anything on it.

That's not how macs work. You don't suddenly lose the ability to run new binaries just because they're not on the latest MacOS version.

Using your own conservative estimate, 8 years seems like a fine lifespan for a laptop, far longer than the "few years" you initially said. And in practice, there are plenty of macbooks that have been running even longer.