Apple's support (to the end of security updates) averages 8.6 years from when the machine was introduced, and 7.5 years from when it was discontinued. So the former figure isn't that far from yours. Here's the data, from Andrew Cunningham at
https://arstechnica.com/gadgets/202...till-getting-fewer-updates-than-they-used-to/
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Not that it wouldn't be nice if they added more time to the security support so it was a minimum of ten years for all devices.
Of course, you get less time if you buy a used machine that's no longer current, but that factors into the market price; it''s part of the reason they cost less.
But you can't expect Apple to count the time from whenever you buy, since that could be anytime. You can buy a 2013 "trashcan" Mac Pro on eBay today. Does that mean Apple should support that machine for another 9 years (until 2034, which is 21 years from when it was current)?