What if Gene Roddenberry committed suicide in 1975?

At first I thought you meant before Star Trek, but you didn’t 1975 is a few years after the original series ended. Honestly not much would be different I don’t think, maybe they would have continued with Star Trek mostly the same way they did now, I believe they already were in the planning stages of a sequel series which eventually became the movie series by then. I also know that Gene had a lot of weird strict guidelines when he started TNG, like nobody was allowed to get mad because it’s a mostly perfect future, and the writers eventually convinced him that they couldn’t write stories if people were emotionless. So honestly maybe the stories would be ever better early on without him instead of taking a couple years to get great. 

Wow, that is a dark question. But I've heard Tolkien was low when he was trying to get his work straight in the mid 20th Century.

   I've always thought Gene Roddenberry was overly optimistic and semi-utopian, but without him, his work would be more soulless and tenth-rate. I don't think he made such a splash in intellectual circles, other people's work wouldn't be changed so much.

Probably the same. Film scripts are written by many people even if only one is credited. If there is a market for something it will happen.