Marilyn Monroe is an icon. You can use her image and say a lot, instantly. Same with the Mona Lisa.
Is it possible to be more of a blonde bombshell than Monroe? Of course you can be better looking or more blonde or more married to intellectual celebrities or dour sports stars. Is it possible to paint a better painting than the Mona Lisa? Definitely.
It doesn’t matter.
Once there’s an icon in place, it’s there because it’s working. It serves a purpose, it carries useful information and performs a valuable function. There will never (or not for a generation, anyway) be the next Marilyn Monroe because this Marilyn Monroe isn’t broken. Countless artists have seen themselves as the next Jackson Pollock, but as far as the lay public is concerned, we don’t really need one, thanks very much.
Google, of course, is the Marilyn Monroe of search. I have no doubt that someone will develop a useful tool one day that takes time and attention away from Google, but it won’t be a search engine. Google, after all, isn’t broken, not in terms of solving the iconic "how do I find something online using my web browser" question.
The challenge for organizations is this: the easiest projects to start and fund are those that go after existing icons. The search for the "next" is easy to explain and exciting to join because we can visualize the benefits. But success keeps going to people who build new icons, not to those that seek to replace the most successful existing ones.