It would depend on what industry you are working in, and the requirements of that. In my job, security is not the biggest issue so firmware and EOL commitments are not top priority. Me personally, I work out what the maximum traffic generated at any one point is, and make sure my devices can handle this within at least 70% of their capability. I only upgrade when a release has been confirmed as stable for at least 6 months, or would plan a replacement device if I can foresee that network traffic/application usage will increase in the near future. I don't like using devices that would go over 75% CPU under load, but that is personal preference.
I think that's a good method, as you don't want devices in the network that struggle under load, however some jobs I go to, they are happy that devices sit at 95% CPU at all times. Although again, it depends on what kind of industry you are working in. If you have requirements to stay within X amount of time of a firmware release or end of life devices, this will be the biggest factor in your decommission plan.
I have seen networks that have devices in them older then me, on the 'if its not broke don't fix/replace it' mindset, and for some companies this works well. I think the ultimate decision is up to yourself, you need to work out what your priorities and your business' priorities are, and work from there.