Saying that all possible ideas for fonts have already been created is far from my own personal truth*. As a font designer who can be inspired by typography in films and video games, and comics, by UX design fake or real, by display errors and glitches, by street signs, by 3D, and a million other things, and the unlimited ways all these can be combined, I find the assumption that there is no need for more fonts because "there are already countless fonts" to be slightly unimaginative.
With this argument, one could argue that there are also countless films, countless books, countless paintings, and what's the point in adding to that? - but somehow, creating is rooted to the human condition and experience (and even some animals create). It's healthy practice, it's a great way to use a brain, it trains your eye and critical sense, and maybe even forces you to achieve something you'll be proud of.
The challenge of creation, whether for fonts or any other artistic form of expression, is to create something new, relevant and exciting that will draw people in. There are millions of font ideas out there that no one's had yet, especially since the technology, trends (and the world) evolve daily to inspire us.
As a font designer and art director, I believe most brands should have their own font as a part of their corporate branding scheme. All commercial products should have a typography-related identity.
Some people create for the sake of creating something new that they find exciting. Some people create purely to make money and don't necessarily have new ideas.
I don't think that going into font design would be a good idea just to make money. Some ideas with no intention to be lucrative have become empires. Some ideas with a desire to be lucrative crashed to the ground.
This simple search with the word "expressionism" seems to return less than 5 good ideas