I love these old ThinkPads. I refurbish and sell them all the time. Just moments before writing this, I finished fixing up a T580; earlier today I did a heatsink replacement on a W510 which is going strong with an SSD and 20GB RAM.
The older they are, the better they are, but even the modern ones are still pretty good. Like the OP mentions, the market for parts is strong and it's easy to get what you need. Then when you go to sell them, they sell for a good amount. That W510 is worth at least $100 in its current condition.
I have a T520. What can I do to make this thing useful again? Even when new, the battery life was pretty awful.
>The older they are, the better they are
Everyone agrees the build quality used to be better (my grandpa already said this about appliances from his youth). But one thing I almost never see discussed is the power consumption of these old devices. Older CPUs often double as room heaters. Modern ones, especially the Apple M-series, have become a lot more efficient. So while I agree that modern laptops suck in many ways, I would do the math to see if it's actually cheaper to buy and use an older computer. Maybe not if you're in Qatar or Russia but some countries have extremely high electricity costs.