I don't think that's "pessimism."
Recognizing reality is an important part of being an intelligent human being, including the examples on Franklin you give.
But a pessimist thinks you can't make things better. I'm sure Franklin still held the belief that slavery would eventually be abolished, even if he was realistic enough to see that it would not be abolished at the time you mention.
A pessimist would just assume slavery and masters reflected a natural condition of humanity, as it had for thousands of years.