With people on quarantine, viruses like flu and common cold, that are less infections, will have hard time to resurge next season. With social distancing, we are unwillingly fighting not only covid-19 but also suppressing the spread of other viruses. In fact, flu season in South Asia normally starts later than in US and with quarantines and masks since early January, this spring flu barely emerged in that part of the world.
Another interesting fact, the flu virus mutates fast, that's why a new strain comes each year as the new "season". With less people infected due to social distancing to fight covid-19, there will be less mutations and we might have several years of mild flu seasons until mother nature create a new strong & infectious strain.