If you go back up the thread you will get the answers.
http://www.nmsr.org/nylon.htm
"It turns out that the novel plasmid's mutated DNA for production of nylonase is almost identical to a non-coding repetitive DNA sequence on the original plasmid; the difference is the single nucleotide that triggered the Frame Shift. This mutation did not exist 60 years ago. If this gene was always there, whether in a plasmid or not, we can reasonably wonder why a bacteria would have a gene for hydrolysing an artificial polymer that did not exist until just a few decades ago; and why, in the absence of such a substrate, was the gene not mutated to uselessness over the millenia?"
It's the mutating to uselessness argument that is compelling. If the mutation were old, then another mutation would have changed it, and that change would most likely have made it useless. If it did improve it, then it would have a higher specific activity than the minimal activity it currently has.
As to surviving on other food sources, all the original metabolic enzymes are still there in the genome. The mutation was in a plasmid.