Gloria Steinem inspired an entirely new generation of feminist writers, humorists and comediennes. Although there may never be another "women's liberation" movement as started by Steinem, there will be another time when women need a voice in government and society. There are already a record 20 women in the U.S. Senate. Perhaps they can take up the mantel of feminism instead.
Gloria Steinem might be a convincing author, but it would be wrong to suggest that no one will ever be (or has been) as effective as her. There are many professors, researchers, psychologists, and politicians that speak on gender studies whose work is just as enamoring to read. Phyllis Schlafly is one of these authors, for example. Schlafly opposes views that Steinem does, but she is just as effective an author at convincing readers.
Yes, as an author there is always room for others to rival you. The reason for her success, or much of it, is because she was one of the first feminists. However, there could always be someone in this area that does better. A feminist author from a strict Muslim country could do much more than she did, for example.
Yes, I do believe there will be. Gloria was a very famous feminist not only for her ideas, but because she came along at the beginning of a new wave of feminism. There are lots of famous feminist voices out there who could definitely become just as popular and effective as Gloria Steinem was, if not more.
Well, I don't really believe that Gloria Steinem's main job or activity was writing, she was by far considered a feminist and activist over anything else. While she did write, it was generally about feminist topics or something that related closely to it, so to say that another author would come around to replace her seems odd, as a fitting replacement would actually need to be a writer, but more importantly an excellent speaker.