This can be a slippery slope with expectations. While the hardware can/might/will function after that time, there is no guarantee on security updates, vulnerability patches, or even a timeline when the system will be EOL'd by Apple and make it ineligible for future OS versions.
Many are assuming 5-7 years of full support, but even that does not guarantee or mean the latest and greatest OS version at the end of that life cycle will work... and you're expecting an additional 3 years beyond that?
I fully get no one wants to be blowing money or budget on "unnecessary" computer stuff and the hacks to keep MP4,1 and MP5,1 going to this point are more than most could or should have expected. The unfortunate reality of the situation is the CPU's are EOL'd and have pretty significant vulnerabilities that are not patched and never will be. More vulnerabilities will be discovered and exploited over time. macOS is no longer officially supported. OC has worked for some, but it is not perfect. Anyone who wants to keep these machines going really should have a backup plan and (at minimum) some kind of outside hardware firewall protections if they want to keep them connected to internet.