There is an open source version Gnash. I installed it on a laptop a few years back and it ran fairly well, but a bit slow compared to the real Flash. Why do you think that didn't take off?

Edit: Looking at the wikipedia page I see one reason (though I am not convinced its the reason it didn't take off). https://en.wikipedia.org/wiki/Gnash_(software)#Adobe_Flash_P...

It's not fully compatible. Of course it didn't take off. You want the default application to be free software.