Personally I see the general benefit of sandboxing as good enough to warrant the large amount of work to implement it system-wide. I do acknowledge that there are a ton of very complicated details around it that needs some good thinking and resolving for the concept as a whole to work well, just as some very clever people have pointed out.
Ultimately I don’t think it’s a disaster or that it will dumb the Mac down. There will be new, different, ways to do essentially the same things that the heavy-duty users, the pros and the geeks have been rocking for some time. While AppleScript is fine and dandy for many things, I don’t think most users even know it’s there, considering how many people are both new to the Mac and “just using it” in their daily lives.
Both Apple and the developers have a big job ahead of them to pull this off and some tricky questions need answers. As usual, I hope they can pull it off in the end.