As another comment pointed out: Toy Story was re-rendered at great effort for 3D. That's the exception not the norm. They didn't feel it was worth it to do it yet again for the 4K version which is simply upscaled.
It is in no way a "download the archive and type make again" sort of situation. It's a huge effort.
Are your comments actually informed or are they just synthesis (guessing)?
> “We had to have some very, very smart people at Pixar go back in and write some software and figure out a way to make it so that those files would render on our current computers,” he said.
>It took four months to resurrect the old data and get it in working order. Then, adding 3-D to each of the films took six months per film.
It is in no way a "download the archive and type make again" sort of situation. It's a huge effort.