You are wrong. You probably never used it. There is no such tools to develop apps like was possible with flash.
The argument that you can do with x technology today the same thing makes no sense. Today you have to be a skilled programer to do the same stuff and it might still work worse today as it did back then.
HTML5 is nowhere near as capable. Webgl . Is there a graphical tool to use it? JavaScript tool? Dumbass Lottie files tools that cost 20$ per month and suck.
And if you were a good programer in flash/flex, you could build apps just as stable as you can today.
It's stupid that tech like this doesn't exist. Except for dumbass Rive - again 45$ per month and it sucks