Is that threshold for using a Pro machine 3 years, I don't know probably not for most people. My M3 Pro from almost 3 years ago is still running beautifully. I could make the argument that more power would benefit what I do with this machine without a doubt, is it worth the upgrade? I don't know, but I would be lying if I have not been thinking about it.
But I also think about the mentality I had when I bought this machine, and the mentality I had when I was looking at possible upgrades just a few weeks ago. Possibly paying more to overspec the machine compared to my current use case assuming that I want to continue to use it in 3 to 4 years.
I don't know if that calculation is necessary as cut as dry.