Serious? It's a paper constructed as part of an official NASA workshop, the participants of which are all respected people in their fields and still working.

Why hasn't more work happened in the time since? It has at places like Zyvex and the Institute for Molecular Manufacturing. But at NASA there were political issues that weren't addressed at all by people advocating for a self-replication programme then or since.

Freitas has more recently done a book-length survey of work on self-replicating machines before and after the NASA workshop. It's available online:

http://www.molecularassembler.com/KSRM.htm

(BTW the same fallacy could be committed against AGI or molecular nanotechnology, both of which date to the 50's but have had little followup activity since, except spurts of interest here and there.)