If we imagined a scenario where Sony and Microsoft left and market AND all third-party games appeared on Nintendo AND Nintendo no longer gimped third-party sales the way they did the last time they had a monopoly ...
Then I think we'd see a shift over time. First, if Playstation and Xbox fans migrated to Nintendo, they'd automatically begin to buy more third-party offerings than is typically seen on a Nintendo system. Second, the buying habits of prior Nintendo fans would begin to change over time as they discovered new franchises, developers, and genres.
The sales of Nintendo games would still be very high, of course, but the percentage among the installed base would see a downward spike at first and then a slower, more gradual descent.
