Your 2012 rMBP does not stop working, just because Apple stops active support/updates for the system software.
The various internal tools that provide day-to-day security of various kinds (SIP, XProtect, MRT, Gatekeeper, etc) do not suddenly stop working, and continue to be updated regularly.
There's no pressing need to retire your older Macs, just because Apple no longer actively updates the system itself.
(You CAN take your old Mac to newer system versions, there's whole threads devoted to installing unsupported systems)