IF Thomas M. Siebel can accurately see the future, computer science students with the entrepreneurial gene may want to look for a different major. And investors who think that information technology is a sector that will produce outsized returns should wake up.

In Mr. Siebel’s view, I.T. is a mature industry that will grow no faster than the larger economy. He contends that its glory days are past — long past, having ended in 2000.