I would still expect to get 20 years of security updates even on Ubuntu right? At least 10 years of major version upgrades and then 10 years of support for that last major release.

Inter Core is pretty old at this point (quick Google got me 2006) and I would expect that it will take quite a while yet before it is hard to find a supported release that have not taken the jump and dropped support.

As you are used to macs, that is a safe bet.

But keep in mind that the lifetime might not be limitied by the hardware. You do not want to be using a internet connected device that is not getting security updates. Apples policy is to only fully support the current releases with security updates. You can get a feel for expected lifetime by looking at what devices did not get the latest update this year. https://www.macworld.com/article/782634/macos-ventura-compatibility.html

I have had to help to many relatives with they old useless windows laptops to recommend going that route.

If you really want to go BIFL, linux is your best option. Ubuntu patches security updates for 10years for each release, and all devices (with some rare exceptions) can get the latest releases. There are also options that are more suitable to really old hardware when your pc can not keep up with the latest features.

If you are going to study anything IT/software/computer science it is a perfect fit. For other fields you will have to check that the software you need will be avaliable. But if you mostly want to browse the web and write essays, a linux laptop will do that for a decade easy.

If you are comfortable with installing the os yourself, https://frame.work is an option very in-line with this sub, they are not tested by time yet though.

Lenovo and Dell sell pre-installed linux. https://system76.com/ and https://www.tuxedocomputers.com/ are dedicated vendors.

A used ThinkPad is a classic budget friendly choice if you are comfortable installing linux yourself or know someone that can help you.