I absolutely love Pico-8, and actually teach that to teenagers at the library on Saturday, and I will agree that it does somewhat satisfy some similar niche as Flash.

That said, Pico-8 is (purposefully) very limited, and if you want to do something a bit more advanced than C64-era gaming, the limitations show. Flash is limited too, but not as much.

It's obscenely fun to play with, definitely agree with you there.