macOS updates in the future

All Macs will become unable to upgrade at some point in their future. No one here can tell you exactly what the timeline for the 2020 Mac will be.

I can tell you my 2012 Mac could still be upgraded to the macOS that was released in 2019; macOS 10.15 Catalina. That is as far as it can go for upgrades. However I am still productively using that Mac every day and it's now 11 years old!

Don't live in fear. There's no reason to be scared. Buy what you can afford now and use it. When the machine eventually becomes obsolete and unsupported, then replace it with another.

Apple has historically provided security upgrades for the current version of MacOS, and the two previous versions.

MacBook Pro 2020 can run Sonoma now. No one can predict with certainty when it will no longer be eligible for the latest version, whatever that may be.

Even if it were on the no-more-upgrades list next cycle (which is completely unknown) then you would still be likely to to get a security update in Fall 2024, and another in Fall 2025.

Not being able to obtain the latest version and upgrades does NOT mean you Mac will keel over, dead when the next version is issued. It will still be able to do everything it could do when brand new, and likely a whole lot more with the free upgrades already provided.

Go ahead and "Run it 'till the wheels fall off."

NB>> be certain you are buying a computer that is completely functional and unencumbered by any locks such as firmware lock, or mobile device management (MDM) lock. Be sure you are not purchasing in an environment where the seller can ship you a useless, locked, MacBook Pro, take your money, and then disappear.

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if you do not edit videos daily, the Apple-Silicon Mac's processing power is so much of a leap over previous Macs, that if your work allows, you could easily step DOWN a step and get increased performance. Apple-Silicon MacBook Air will outperform that 2020 MacBook Pro for all except possibly video editing.