I don't think the population of the city of Oakland itself is relevant. There are smaller cities that have no problem supporting teams because they are in large single-team markets (St. Louis, for example, is about the 70th largest city).
It is very reasonable to ask whether the Bay Area can support two teams. The other three two-team markets are in the 10 to 20 million population range. Even if you include the San Jose/Silicon Valley area, the Bay Area's population is only about 6.5 million, not much larger than the Philly or Atlanta areas, and smaller than DFW and Houston.
The stadium makes a difference, and the Coliseum is one of the worst. And the team is not consistently competitive though even that cannot overcome the stadium issues (as in Tampa Bay).