...why not pay now for a show that gets 3 to 5 million veiwers now and makes a small profit. In order to make a larger long term profit from the above mention sources?
It's yet to be demonstrated that a Trek show that gets "3 to 5 million viewers" would turn even a small profit for the studio in the short run.
Also bear in mind that the studios choose between many projects every year, and they do have limited resources. If you can produce six or eight sitcom pilots for the cost of a single Star Trek pilot why would you go with Trek, given that you're more likely to have a hit with a sitcom than with an expensive science fiction melodrama.