Do you have a source for this alleged temporal decay of efficacy?
I searched google.com for "increased antibiotic resistance over time" and found two sources that seem pretty legit. One is the World Health Organization, the other is the US National Library of Medicine. Both articles appear to address the question I believe you raised.
[0] http://www.who.int/mediacentre/factsheets/fs194/en/ [1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4378521/