Stencyl seems like a great program in my opinion. They have the right idea about supporting flash and multiple platforms also.
Flash right now is more powerfull then html5. But the focus is on HTML5 tech (plugin-less in general). The big boss companies are betting everything on it, brilliant devs are makes brilliant things on it, there's tons of engines and frameworks for it etc etc.
As you say Flash is a lot faster, if HTML5 uses WebGL for the GPU boost and it's running in a decent browser for some things it's a bit faster than well coded flash, however if you then enhance flash also with Stage3D, Alchemy etc then flash totally destroys HTML5 in terms of power and speed.
Being a general coder using a range of programming languages i recently decided to go back to code with AS3 instead of using C2 for the moment. Don't get me wrong here i mean C2 is great in my opinion and why i purchased it but i don't think HTML5 is as good as flash yet so i am basically just waiting and focusing on other things for now. HTML5 is also constantly chagning at the moment, i made a few plugins for C2 a while back but had to make adjustments with new releases and then got hold up by needing updates to the SDK, months later they are still not added probably due to the developers having a load of other things to add first so that is also another reason why i have decided to change focus but then return to C2 at a later date.
Adobe have already announced they are dumping Flash in favour of HTML5 for mobile devices. Not sure what plans are for continuing for PC or MAC. With severe stability issues surrounding Flash lately... Adobe will need to do do something before HTML5 not only catches up, but far surpasses Flash.
No that is not correct, firstly they have not dumped it and will continue to release bug fix updates, after they released V11.1 they now are instead focusing on Adobe AIR and HTML5 for mobile devices because they are better suited for them. I am guessing they don't want to focus on flash because it would be a load of work having to make updates everytime a new phone/tablet was released which had a different configuration just so flash would work on it, with Adobe AIR and HTML5 they would not have to worry about that however.
Adobe now owns the Phonegap service and makes other tools like Edge so with HTML5 they probably want to stay current and also to try and start making money with these also. That does not mean they plan to stop all support for flash though but multi platform is a good way to make money and also if Flash ever did end many (hopefully many, many) years from now they would then still be able to keep going with their web based software.
For PC flash however support is still very strong and they are apparently now working on Flash Player 12, with V11 it just got Stage3D which is much better than WebGL in my opinion also and faster.
So while many pro HTML5 supporters like to point out about the mobile thing, what some of them say is often not right correct and a very powerful version of flash is still avilable for many mobile devices and for the PC platform flash is definately not going anywhere for a long time. Plus there is so many flash games online already and it's not like as soon as HTML5 was released people stopped playing them, if games are good then people will keep playing them and that is also true for other languages like Java with Minecraft etc.