I have a pair of SF Il Cremonese acquired Jan 2018. The silk strings began sagging on all 3 surfaces over the last 12 months. Conversely I have a pair of Guarneri Evolutions dating from 2011 and they show no sign of sagging. Strings are still taunt.
I was hoping that perhaps moistening them then letting them air-dry could restore them as I do with some painting on canvas when the humidity reduces their surface tension.