Your confusion in this thread stems from the assumption that there is only actually enough CS to cover 4 years for a good student and 5 for a mediocre bs+ms. This is complete bullshit because the truly motivated undergrad would spend the senior year doing 500s and then the next year doing a bunch more for more breadth.

At any of the large CS universities, there are usually 4+ masters tracks you can go down (security, hpc, p/l, HCI, algorithms, etc).

Someone who quits with just a BS has no interest in going further theoretically. The absolutely did not cover all of the topics available - let alone get into research.