Gene R. created ST, and honed the concept with TNG. Even when TNG improved in S3, it did so with the elements created/approved by GR. Trek isn't just a proven, brilliant story format, it's an inspiring vision of the future, special enough so that by the time others tried that tack, they just seemed pale copies of Trek (e.g. Buck Rogers '79, Seaquest).
He was a credit thief? Welcome to Hollywood! That's how things are done there. Not to excuse it, but he was hustling to be a successful producer in a brutally competitive industry. Saintly types don't get their shows onto network TV. Rod Serling, Irwin Allen, Glen Larson all get the same charges.
Also true: he was a better producer than a writer. However, he apparently did a lot of re-writing, including of TOS S1 (season of seasons!) and that may never be credited. Fans complain that he left TOS, then complain that he showed up for TNG? Give me a break! His work speaks for itself, and the franchise reflects Roddenberry's vision more than that of any other individual. He had help, but if he wasn't around: no Star Trek.