If you had to point to a specific moment, the 1997-98 school year seems like an obvious one.
The winless football season in which all of the chickens of Guentherism came home to roost exposing how far we'd fallen behind modern college football immediately followed up by a Big Ten championship basketball season under a new coach pushing us forward beyond the Henson era. And of course also signing a recruiting class of Frank Williams, Robert Archibald, Damir Krupalija and Lucas Johnson.
That was the moment where it was unmistakably shown that basketball's success in the 80's had durable legs whereas football's didn't. The gap of which program is more worth your time to care about has occasionally narrowed but never come close to closing.
(Ubben was also built that year, seems relevant to note)