Honestly the 2015 model isn’t that bad processor wise. You’d easily get 6-7 years of heavy, quality use out of that machine. Ask most people still using their unibody models from 2012 if they have any major issues, and most will say no (except for the few with one of the issues those models had). Apple does a decent job keeping up the software so that their 5-7 year old devices can keep running well, IMO.The processor is pretty old, I want to get something that’s going to last for a while.
I only upgraded from my 2012 15” MBP with a 2.7 i7 to a 2017 13” MBP w/TB because I needed a machine that natively supported dual external monitors, otherwise that machine was still fast (I did upgrade the harddrive to an SSD).