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Microsoft tries to convince Windows 10 users to buy a new PC with full-screen prompts
(www.theverge.com)
This is a most excellent place for technology news and articles.
If your Air hadn't reached it yet, eventually it'll reach EOS and you'll stop receiving software/OS updates from Apple. While this won't force you to upgrade hardware, it does add significant risk to your online time, since vulnerabilities will go unpatched. But, again, an important difference, which you shrewdly point out, you're not forced or coerced to upgrade.
Edit: autocorrect
Fortunately, Firefox still supports old Macs. I have a running 2011 MacBook and 2008 iMac that are both still used for 3D printing and the other for an old local version of accounting software and email respectively.
While true, the vulnerabilities I'm referring to are those in the OS itself. But if they're not connected, you should be ok. All I'm saying with this, is be super careful. Some of those vulnerabilities are zero click.
Edit: autocorrect that killed my grammar.