Your last paragraph is my point. The inference is that because a certain person is involved, ‘X’ will happen.The statement "Being TC doesn't guarantee a good speech will be given" is a strawman. Of course, things could go south, but TC has several high profile universities under his belt, and with that, Gallaudet is taking a measured risk his speech will be well received. But you are correct, in suggestion past performance doesn't guarantee future performance. And narrowly focusing on that as a talking point, rather than the several successful speeches TC made is folly.
Nothing in life is guaranteed, but the smart money says TC will give a successful speech.
But again. Joe Public is often bedazzled. If someone got up on stage directly after TC, regardless of what they thought about TCs performance they’d very likely thank him and also say something along the lines of ‘insightful and enlightening words’. It’s just what happens in life.
Celebs are very useful in that they shine a spotlight on things regardless of how good their actual input is/was.