Jan summed it up nicely. The only thing I could hope to add to that is to reinforce his observation about FoxPro, and how that relates to other products like HC and Rev.
HyperCard was owned by a rather sizable company, as was Gain Momentum (a Unix-based xTalk back in the '90s) which was owned by Sybase. Same with Oracle Media Objects, which was available for Mac and Win in the early '90s.
Apple, Sybase, and Oracle have so many things going on that they can afford to drop products like their xTalk offerings without blinking. And after just a few years, they did.
RunRev Ltd. has only two products, Rev and Ten Thumbs Typing Tutor which is made with Rev, so they're fully committed to seeing it through. They have no choice, and personally I like that. Bigger companies can afford to be fickle, but RunRev Ltd. is fully focused on expanding the Rev technology and its user base.
As with any sole-source proprietary development system, it takes a long time to get traction, certainly longer than most open source systems. If Flash hadn't been acquired by Adobe who knows if it would be as popular as it is today, but it seems unlikely. With fewer marketing resources than Adobe's multi-million-dollar budget, RunRev Ltd. has to take a more patient path.
But that path has been delivering reliable, solid growth year over year since they first started selling Rev back around 2001. With v4 offering a free RevMedia for development, a free RevWeb plugin for deployment, and soon a nifty new free server component, the options for growth are multiplying.
As with any company, there's no absolute guarantee that RunRev Ltd. will be running in perpetuity. We could all be hit by an asteroid tomorrow.

But there are enough developers and their clients building businesses around the Rev technology that the larger Rev ecosystem has plenty of money floating around to keep the code base moving forward for a long time.
I occupy a very small corner of the Rev ecosystem, yet just among my own products and those of my clients we collectively have tens of thousands of users with combined revenues well into the millions. Multiply that by the number of developers making other commercial products with Rev, and the scope of the Rev ecosystem becomes clear.
Additionally, RunRev has noted Mike Markkula, former Apple chairman, among their many investors. Between good funding and guidance from people experienced in this sort of technology, and the growth opportunities for the many free Rev 4 products, I'm pretty confident about the state of things for the foreseeable future.