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.
This is probably the case for a lot of people, but Apple is smart with their lineup and there are several what I'd call "not pro" features that are gated by the MBP (and iPad pro) which is unfortunate.
Primarily, 120hz screen, SD card reader, HDMI port. None of those are really "pro" featuers, they're just basics I'd expect on any premium laptop.
That being said, 3 years is a tad on the aggressive side for an upgrade cycle for a laptop. I just upgraded my M1 last year, and this M4 Pro will easily last a year or two longer before I feel a compelling need to upgrade, outside of getting RAM constrained. Use case is photo+video production. So for me, a 5 year cycle would be about right.
But also depends on how you spec it out. If you are just buying the base model, then 3 years makes sense. If you spend extra to max it out, you'll be able to hold onto it for 6+ years.