Flash really was that easy for so many people. Why isn’t there a true replacement?
The technical legacy of Flash made it a poor fit for touch screens. Apps filled that void with astounding speed.
This part is just a vague opinion I am riffing with - With mass available video, a ocean of video content may have changed tastes enough that the demand for interactive content diminished somewhat. Social media also killed of the dedicated website for a lot of places that used this technology and thus the demand for this stuff.
Once Google via Chrome closed the Flash hole, I do wonder if they will ever let anyone else to try and get in on that space? Time will tell.
Unity kind of took its place in the game space.
Web pages - nothing. They're all using some boring non visual JS library.