Your letter mirrors the stories that I've heard from many others. Since the HyperCard article was posted, I've also been informed of many HyperCard alternatives that will run on Mac OS X. But none of them have been as complete as HyperCard itself.
I think that a possible Mac OS X solution to replace the original HyperCard has to have these three things if it's going to be embraced:
1) People must be albe to port their existing HyperCard stacks to the new application.
2) The new application must be affordable.
3) The new application should be easy to script, and if possible, feel as much like the original HC as possible.
That doesn't mean that the new HyperCard can't be modernized, but not if it means losing its original strengths.