Whenever I do embedded work with counters, every time I assign a variable that has a possibility of overflowing I do a little mental count about how likely it is to overflow. That's part of of the software development process.
They may not have had a meeting about it, but I think it's exceedingly unlikely that whoever decided to assign a 32 bit int to store time didn't give some consideration to the date range it could represent. Otherwise how would they know not to use a 16 bit int?
They didn't design it in a vacuum. They had worked on other OS'es already that used 32-bit ints with a 1 second quantum and they (probably subconsciously) thought that if it had been good enough for those other systems it was good enough for Unix.
They may not have had a meeting about it, but I think it's exceedingly unlikely that whoever decided to assign a 32 bit int to store time didn't give some consideration to the date range it could represent. Otherwise how would they know not to use a 16 bit int?