I have a FW13, 3 yrs old, battery was getting weak, i just ordered one from FW and popped out the old one and put in a new one. Same for SSD and memory. This alone makes me stay with FW.
Just did it with my old Dell a couple days ago – I was done in 5 minutes.
But if it lasts 3-4 years I guess many end up upgrading anyway. My Latitude 5330 has everything soldered even the disk but going strong for 3.5 years.