You haven't addressed the biggest concern about Windows updates - no AV vendor can detect missing updates for an OS which doesn't receive them any longer.
Not nessisarily. Most AV vendors have working knowledge of vulnerabilities discovered and can actively deduce how those issues will affect legacy OS's. Many did this with XP until the differences in code made such effort difficult or irrelevant.
For example, Avira was especially adept at protecting XP with unofficial signature updates up until 2017 when they ceased support for the OS entirely. In fact, they have publicly committed to support of 7 until 2022;
https://www.avira.com/en/support-product-lifecycle
That's just one vendor.
Comodo
still has a free AV that works with XP, though it's functionality is limited, it still does a great job.
https://antivirus.comodo.com/security/free-antivirus-xp.html
Comodo's Personal Firewall also still has a version which runs on XP that is robust enough to protect that OS from the vast majority of threats one might encounter on the net.
I personally still run a machine with XP on it that I occasionally get on the net with. No virus attacks or hack attempts. It's one system in billions hidden behind two firewalls and I do not go to "IShouldNotBeHere.com" types of sites.
Microsoft's support for Windows 7 may soon end, but third party security suite support will likely continue for years to come and will continue to be effective. Windows 7, like XP, will not just fall to pieces overnight, or at all.
Also, you haven't addressed the fact that MAC addresses can be easily spoofed especially when you're on a wireless network where there are no physical ports you can assign them to
True, but there are ways to block such problems. For example, transfer of files manually through external storage(USB drives) or isolation of devices to specific subnet addressing.
In short, you either choose security or obscurity via trying to patch your holes and having the faith you haven't missed all the attack vectors an insecure/unsupported OS opens itself to.
The third option, I've already mentioned, is to hide the OS in question behind protective security measures and proper security methodologies.