Lucas was asked about the financial stability of Lucasfilm after 1983. Lucas said, "In the early 90's, we were breaking even every year, and I was presented a proposal of how much money we could make if I made the Prequels. If I made the Prequels, I could be financially independent for the rest of my life, and make any movie or project I want through Lucasfilm, not the studio system. If I let SW go, and stopped after ROTJ, then every project I would do in the future would hinge on the studio system, as Lucasfilm was not some big money maker before the Prequels were made, where I could essentially do what I want now."
I believe the PT was a safe way of Lucas never having to worry about the Studio system ever again, whether it be the SW Sequel Trilogy or anything he wanted do, so from a business point of view, the PT was a success. But anyone who wonders why TPM was kiddy, why it never seemed to hit the tone of ESB in any PT movie, was essentially Lucas had to reach the maximum amount of fanbase he could, cause in the end it was about money, not quality.