If it were to happen (not saying it will), but if it were to happen it would probably happen like this:
1.) Smartphone games becomes a big, maybe even central part of Nintendo's profits on a yearly basis. Single smartphone hits can earn up to $1 million/day, it's not a reach that maybe 5-6 different Nintendo IP could rack in $300-$500+ million/year for Nintendo. That would show there's a large market of people who want Nintendo games outside of just Nintendo hardware owners.
2.) The NX doesn't do so great and their traditional hardware model continues to decline. Handhelds continue to have their market base eaten away, they're unable to come up with a miracle gimmick for their next console variant/iteration, etc. In that case unified platform will help them but it won't be a silver bullet that saves them.
In that scenario I think Nintendo would consider stepping down from dedicated hardware to push their IP to the widest possible user base.
I guess the crux of the situation is this: are people willing to carry around a seperate dedicated gaming handheld in today's day and age ... and the answer to that is increasingly becoming no, so that's a big problem. If they don't have a bedrock foundation with portables to lean on, their track record with consoles is way too spotty to bank the entire company on. Even in a Fusion setup, the handheld has to be the one that carries the load and drives adoption.
