Nothing wrong with any of that if it is doing what you want from it and that appears to be the case.
I doubt that security is as risky an issue with OSX as it is with Windows, where updating the OS version is essential once they stop providing patches for whatever version the user is on. But even with OSX, Apple periodically issues security updates within the currently supported versions. I think Apple stopped doing this for 10.6 with the release of 10.6.8, which is quite a while ago. So not to admonish, but just to state a possibility - there is probably some low level risk of infection; not sure I would lose sleep over it.
Apart from security, the main reason to update the OS is when it fails to support new application or new application versions you want to use; but that isn't your case. It was mine, so I am on El Capitan.
As for the hardware, I'm still using my MacPro mid-2010 version because it just keeps going, and it's loaded with 24 GB RAM and 24 virtual cores, so performs very well. I shall carry on using it until it won't support new software that I may need, or it crashes beyond economic repair. So far I have to say the quality of the hardware has been very reliable.