I don't think everything has to be "modernized" and "updated." When I look at software from the 80s that is still with us, I think: "This is robust, keeps working, and has withstood the test of time" not "This must be changed." I still use C and the standard C library because I know how it worked in the past, I know it works today, and I know it will work for decades to come.
(minus the known foot-guns like strcpy() that we learned long ago were not great)
In contrast when I see software from the 80s that is not a security and performance disaster it’s because of continued investment and most of the basics like string handling have been replaced with bespoke or third party libraries and IO heavy lifting is done with OS specific interfaces anyway.
This is all well and good, but just because something came from before doesn’t mean it was a good idea then, or especially now. You’re basically citing survivorship bias. Of course something still used from the 80s is well made, otherwise it would have been replaced 30 years ago.
Most of the networking stuff from the 80s at least wasn’t particularly well made, it’s just been maintained and significantly reworked to not have massive security vulnerabilities.
(minus the known foot-guns like strcpy() that we learned long ago were not great)