The time_t limit in Linux is inherited from UNIX, which baked it in circa 1970, back when it was developed on PDP-8s (manufacturing lifetime: about 50,000 machines total) so Ken Thompson could write a space wars game on an unused machine with no OS. And then migrated to PDP-11s circa 1972 when someone at Bell Labs wanted a typesetting system.

Back then, digital computers were only 25-28 years old. There was no such thing as legacy hardware back then, in our terms. Designing for a 68 year lifespan -- nearly three times the entire duration of the industry to that date -- probably seemed excessive!