I think there is an important missing caveat. Yes, everyone might be really bright and have 4.0 GPAs, but there are signs that you might have a real chance. Are you not just bright and have a 4.0, but did you also get into more than one of the top 5 departments in your field, giving you the pick of the litter and some negotiating power when you enter? Is your prospective advisor high profile and genuinely excited about you? Are you entering the program with an NSF GRFP fellowship or a Javits? If you are answering yes to these kinds of questions, you will most likely be able to get an academic job at a research university or a major liberal arts college. Seriously. I fit into this profile, defended in 2012, and have a tenure track job. I was in a small cohort--three people--but two of us have tenure track jobs and the other is in a high profile post-doc. I looked at all the PhDs from the last 5 years of my program. 8 people have TT jobs, 1 has a post-doc, and 3 have industry jobs, but 1 of those 3 really wanted an industry jobs when he entered. No one is unemployed, and we're talking about the roughest years from 2008 on.

Yes, it is not the case that a PhD guarantees you an academic job without reference to your department, thesis topic, and advisor. Do people actually believe otherwise, though?

Can you be a professor? Yes, but not based on your undergraduate performance alone. You need to do an honest assessment to see if you are on a trajectory to be one of the top people in your field when you graduate. You can see this, in part, by looking at what the current graduate students in your prospective program are doing and where the recent alumni are. If not, then you should absolutely not go into grad school thinking otherwise.