Wow you were pretty close to going! But I understand you, the same thing happened to me while I was preparing my trip although once I was there I felt there were several things to do and visit. I wouldn't tell you to spend a week in the city, but Frankfurt is a gateway to many interesting nearby places, I got there because I planned to visit a part of the black forest.

And if you are interested in the gastronomic aspect, the truth is that the city surprised me, there is a lot to eat and drink, I missed many things to try and I ate a lot haha.