Development on C2/the runtime would not stop completely - it would just slow down. There's a steady stream of new browser features, compatibility changes, platform tweaks, bug fixes etc. that regularly need addressing, and we would keep doing that. There may be minor new features as well, but it would be more like a maintenance mode than a steady stream of new features.
I don't think the sales impact would be anywhere near that bad. People don't hold off buying Office 2010 because they knew Office 2013 would inevitably show up a bit later, right? If you need to do something now then you'll have to look at the options that are there now, and it's still a great product with loads of features. I am confident despite the scope of the idea it would not come close to "killing" Scirra.
I regularly shoot down the idea of 3D and I'll keep doing it - I think it would be a totally different product, and as one of my linked posts said, it seems to me kind of paradoxical to have all the complexity of 3D and still try to make it simple. Modularity is a very compelling feature, but is a huge undertaking and would probably be easier to code in from the ground up while reworking the editor.
Anyway, don't get too distracted by particular technology choices or business plans - the key question is basically when to do a new editor. Also as
Fimbul mentioned it's hard for outsiders to take in to account everything that we do internally, but the feedback from real users is always useful. I guess it comes down to: do you want a dream UI with everything, but after a long time, or for slow and perhaps painstaking updates to the existing editor, but arriving sooner?