ThemeForest and CodeCanyon are highly successful but they haven't done anything to prevent others from also being successful. Their presence alone is not going to prevent others from being successful.

If anything they have validated that a market exists and it's a market so big and diverse that nobody is going to own it.

A perfect example is Creative Market. They've been successful despite the presence of ThemeForest and they aren't even a year old yet.

The fact that there are big, successful competitors in a space you want to enter should never deter you from doing so if you think you can be successful.

Brad put a ton of work into this project and he learned a lot in the process and ultimately he came out of it with a new product that he is enjoying success with. Not every project is a success. You learn from it and move on.