Worth noting that while Hypercard was the glue which held everything together, most of Myst's functionality was implemented in native plugins. (or at least that's what Robyn and Rand told me when I chatted with them at a MacWorld Expo around its release). Their earlier games (The Manhole, Cosmic Osmo, etc) had been much more pure-Hypercard affairs.

Interestingly, for Riven (Myst's sequel) they were still using Hypercard to author the game, but the game no longer ran through Hypercard; instead, it ran on a custom C++ engine which ran on the data files output by the Hypercard authoring tools.