When I was a student in the 1960s, the prevailing view among scientists was that life on Earth was a freak phenomenon, the result of a sequence of chemical accidents so rare that they would be unlikely to have happened twice in the observable universe.No, Drake said in 1961 that there could be millions of civilizations in our galaxy. The evidence and arguments for extraterrestial life have not increased much in the last 50 years.
I also don't agree with his reasoning that microbes may be improbable, but if there are microbes then they probably evolve into sentient life. My hunch is that the reverse. I think that it is plausible that microbes are common in our galaxy, but that they have not evolved into sentient life anywhere but Earth.
Life started fairly early in the history of the Earth, but we have no idea how it happened. Maybe it was a freak event, or maybe it would have happened on any similar planet.
We know a lot about the evolution of life on Earth, and intelligent life is a byproduct of a long list of freak accidents. It seems very unlikely to me that all those accidents would be replicated elsewhere in the galaxy.
Update: In the 1980 Cosmos TV series, Carl Sagan also used the Drake equation to estimate millions of advanced civilizations in our galaxy.