I'm going to address your question in two parts:
1. The accuracy of the claim that attendance dropped.
2. The effect that the lectures had on the attendees.
Firstly, as you rightly hypothesised, the tale of dropping attendance was false. This stemmed from a preface to the book, written by David Goodstein and Gerry Neugebauer. They wrote that
"… as the ...
Jean Froissart wrote
And I often, with a little tube, Made a soap bubble fly from the
I'm not sure when he wrote this but he died in about 1405.
Also, there's a picture of a monkey maybe blowing a bubble.
The text under the picture says
Detail from a full border of a monkey playing a game (possibly blowing
bubbles?), from the Isabella ...