I am currently reviving an T410 for my kids. I put an 250 GB SSD inside and the newest Linux Mint and play around with it now. I am still on 4 GB Ram, as I didn’t want to spend the 60€ to upgrade to 8 GB, yet.
It runs great. I can watch YouTube, browse the web and rip some of my CDs for my NAS and my Kids Audio Players with that sweet internal DVD drive. My guess is 60% of the people would not need more computing power. And this machine was released in 2010.
I would try the „retina formula“ to see, if the upgrade would benefit me.
Basically apples retina displays are engineered, that either the selected pixel density, pixel size and typical viewing distance, single pixels cannot be seen by the human eye. See https://en.wikipedia.org/wiki/Retina_display?wprov=sfti1#Rationale
If you have a small tv and it is several meters away from you, my guess would be that the difference is not that big.