Well, our license agreements with publishers are structured in such a way as to ensure that your rights to use a dictionary are independent of (and outlive) our rights to keep selling it. So a reduction in our dictionary portfolio would not affect your purchases.
As far as our more basic survival, though, aside from the fact that we've been in business for longer than Facebook, we aren't really able to *legally* guarantee anything because we can't ensure that our app will keep working forever absent someone maintaining it; whatever succession plans we might have, nobody's going to promise to keep updating our app forever if that turns out to no longer make financial sense.
As a practical matter, the cost of keeping our app working on new iOS / Android updates is small enough relative to our revenue that it's extremely unlikely someone would not be willing to take that on; if I die or become a monk tomorrow there would be all sorts of people interested in taking over Pleco. Aside from the cost of developing software updates updates, our server hosting costs are minimal, and maybe 80% of that small sum is bandwidth for people downloading our free Android app outside of Google Play (something that we could easily limit to paying customers if we needed to).
But if, say, we encounter a situation like 2008 where Palm and Windows Mobile both abruptly died and we had to rewrite our app mostly from scratch for two brand new platforms, it's quite possible a post-mikelove Pleco would not survive that; then again, neither would most of the other apps you've purchased (certainly few Palm/WM apps are still going now), or probably even much of the other digital content.
Assuming I do not in fact die or become a monk, though, my youngest kid is 16 years away from college so I have a pretty strong incentive to keep this very flexible business I can do from home working until then