I've gone from the A21 to the T42P to the W701 to the P71 (last year)
My general criteria for an upgrade is a better than 2-to-1 improvement in memory, storage capacity, screen resolution, and speed. Usually I get around 3-to-1. These have all been solid machines, travel a little bit but are also remoted-into.
Every time a new model comes out it has a different keyboard layout. In my 20 years of ThnkPad usage I get accustomed to a certain fingering. The P71 layout is the most problematic and I have to keep hunding for the Delete key.
I've slowly moved all my laptops (including a bunch of R models) to Linux (Mint) with great outcomes.