I happen to have a live example, which taught me a good lesson. Before that, I 100% agreed with you and I followed the your idea myself actually. However:
1) Knowing most of the product and all the skills needed and with enough management skills does not mean you will become a senior too. It also depends on from which angle to know the product and the vision of the whole area, the latter is more important. In many company(if not all), PM did not have concrete idea in the beginning, he needs input from developers.
2) One of my friend (not IT majored) studyed J2EE in a few training school. He knew concepts very well and had almost no coding experience except his homework. If he applied for junior job, he will sure be tested detail coding and will fail. However, he applied senior directly and presented himself very well (His english is good, and he is always over-confident). Then he got the job. From the beginning, he often asked friends (including me so I know how poor he was then) to help him in doing detail programming. But nothing is really difficult if spent enough time, espeically in java. Sooner he picked up. A few years later, he became a real expert in EJB. BTW, in many companies, seniors do not have to be a leader.
You might argue he's too lucky. However, investigating his story, I noticed some key facts that lead to his success. -victorzhou(patty) 2006-6-10