You are making good points there .
However, a 'pro' workstation's usability is measured by its potential, not by the needs of individual users .
In every other field, no professional would ever consider investing in a tool or system that is not easily adaptable and expandable ; it doesn't matter whether or not upgrades or expansions are required right now, but they must be available .
The MacPro 4.1/5.1 goes back to 2009, and is still competitive today - granted, it will require a few upgrades for the older models and a few things won't be available , but the potential is there and it's fairly easily done and affordable .
I don't know of any professional that deal with upgrading internals of their equipment, they replace the entire thing. It makes no economical sense to deal with upgrades as the money you are saving compared to replacing will be eaten up by the downtime anyhow.