You can bet your bottom dollar that devs will embrace the new non-destructive plugin model. It's a much better way of doing things.
It all depends on the level of editing in this new app, you mention yourself there's scant information. Many people are jumping to the conclusion that all the plug-in makers are frothing at the mouth over this. We'll have to wait and see how much editing capability will be in there and what apple will allow, for plug-ins.
I agree with the cloud and the cost, though I think it will be the default location just like many of its other apps. Apple will be pushing cloud storage over local storage, for the very reason you are complaining the cost, i.e., apple see this as a revenue stream.