It is within the realm of possibility!
Much like Nassar, Bill Clinton, Bill Cosby, Harvey Weinstein, etc.
One would certainly think that with the amount of media attention, with the stakes at hand, that somebody would’ve come forward by now. Obviously a very hard thing to do, but as recent history has shown us, something that has happened in recent allegations against high profile people.
Some could argue that the lack of other women coming forward is some type of evidence. I don’t think I would argue that, but some people could.
Anyways, Feinstein didn’t ask the FBI to look for other cases. The FBI was asked to look into a specific case. Your argument doesn’t hold up.