My 2c, just to add to the discussion is this.
Why construct: Scirra knows it's tech, the visual editor is first class, no other visual programming toolkit out there compares, for development(non-educational purposes). It's amazing, or ridiculous, the amount of out-of-the-box functionality that you get with Construct. And the performance is pretty decent and getting better. Seriously, i've benchmarked game projects made and tested in multiple engines in the past, and construct was one of the great winners in the html5 arena.(not sharing them, don't ask)
The one thing that construct has against it, that i think scirra doesn't see, and it really is it's Achilles heel, is the asset management pipeline.
People will use other engines/frameworks(phaser, cocos, panda, etc) because there they want to control when things are downloaded, how they are processed, and in which way they are presented.
It would be great if construct had something like Unity's Asset Bundles, or at least some way to defer downloading assets til later. As other devs have suggested, some pros want the ability to control what goes into each generated sprite atlas, for instance, and other more personalized controls over how assets are processed. And some other individuals would also like to transform these assets at edit time and runtime.(I think this is coming recently?)
These are changes that I'm sure the new runtime can accommodate and that are a lot simpler to implement that all the features Scirra has been implementing that relate to performance.
I hope they take that into consideration and make the engine more pro-friendly. A lot of people wanna use Construct but they need the right functionality for bigger games, and certain audiences.