Assuming they realize that they need to catch up to modern standards or risk becoming irrelevant, i agree, they'll probably be involved in gaming for the longest. It's their bread and butter after all. I don't expect any of them to leave for a very long time though.
That said, they aren't as invulnerable as the OT implies (intentionally or not). As the 8th gen has shown us, Nintendo are very much as the whims of the market as a whole. Their lack of relevance to the mainstream home console market, as well as the rapid growth of smart phones have resulted in some of their worst financial results in decades.
You 20m comment is somewhat baseless though. Nintendo have never sold only 20m units in a generation, and without them giving us a proper revenue break down of the GC we don't know if they could sustain themselves on an install base of only 20m. I highly doubt they could. They're too big to survive off a revenue steam that small (not without downsizing, anyway).
