It’s safe to consider superheroes in their current form as a fad. Even if they’re been around before, most of them didn’t get big until the late 20th century. There were Punisher, Captain America and Fantastic Four films before especially in the 70s to 90s time frame. You can go as far back as the 40s with the Shazam, Batman and Superman movie serials.
X-Men have been quickly catching up with Spider-Man and Fantastic Four since the 1980s. The Flash too has been on the slow-burner despite being much older than me though the question is how fast the Flash catch up to become a big franchise on par with Superman and Batman.
I don’t see much merchandise for the Flash programme. There’s only merchandise for the Flash comics and that’s usually relegated to toys, fast food flyers and t-shirts. You could say it’s too late for the Flash to become a big franchise given the possibility of audiences getting turned off by S2 similar to what happened in Arrow S3.
Arrow suffered a lot after pandering heavily to a narrow but vocal fanbase. Most of the comics fans don’t like and so do the general audience. Its ratings plundered and original direction went nowhere. The biggest misstep for the Flash would be to turn the protagonist into a bass-wielding detective ala the Clash after he loses his powers.
Flash S2 would be filled with Clash references and would predictably promote the band heavily. Two of Paul Simonon’s sons could appear at any point. Then the love for the Flash dies down so badly it results in a backlash against not only the Flash Clash but also superheroes in general. The Flash will become the ultimate example of cynical marketing. When the Clash take interest in sharing the franchise, expect the Flash to appear in music festivals and appear alongside proper Clash merchandise.
However if the Flash dies, the question is what will replace the franchise? At the very least it would lead to crime novels with a similar premise and protagonist. S2 has yet to come out but would spell the end for the Scarlet Speedster.