I recently bought a 16GB DDR3 upgrade for my T420, purchased in 2011 from the Lenovo outlet store. Sure the bezels date it instantly and the battery life can't compare to an M1 Macbook, but in many ways it's the last "real" Thinkpad that's usable.

It's the last model with the old-style 7 row keyboard, and when my display cable flaked out in 2012, IBM(!) was still in charge of honoring the warranty. I got a box shipped to me that I put the laptop in, it got overnighted for repairs, and overnighted back. It's a chunky and hefty device by today's standards, but that's part of the charm. Swapping in a battery feels like reloading a weapon (especially the 9 cells).

16 GiB is usable in 2022, with exotic upgrade options for CPU (you can go to Ivy Bridge via a BIOS flash (!)), and external GPUs are an option as well (PCMIA !!!). There are 1080p screen retrofit kits, with the main liability going forward probably being the reliability of third-party 9 cell batteries.