I always thought the cutoff should be 9 years for base operating system support, and another two years of security updates. That seems like a long time, but the problem with Apple is that support is all or nothing. It isn’t like other OEMs where new operating systems can be installed regardless of driver support from the manufacturer.I’m all for keeping old machines running as long as possible, I have a G3 tower that I still play with from time to time, but there must also come a point where the OEM is no longer required to support something. I just don’t know where that point is.
Thankfully the patcher developers have found ways around this and have done an amazing job, but I always feel uneasy relying on third parties just to use the operating system my Mac is capable of using.