Lack of Vendor Support
When a device is known to be out of vendor support it will be marked within the platform as "unsupported". A lack of vendor support implies that fixes for newly discovered security issues may not be released. This will likely lead to the risk of running these platforms from increasing overtime as more issues are found but not addressed.
Additionally, several compliance requirements such as Cyber Essentials will consider an unsupported operating system an automatic "fail", for the purposes of compliance.