Exactly, it's Nintendo's job make the system atractive to third parties.I guess the real question is why third-parties would be incentivized to add additional investments in Nintendo platforms when they're theoretically already peaking their franchises' performance just on Xbox/PlayStation/PC. It's not like Nintendo customers represent millions of people who actually increase the install base for their games, otherwise Nintendo customers would have bought the Wii U ports because that would have been the only way they could have played them.
If Nintendo customers are already customers for their games, they are customers for their games on other platforms. In which case investing in NX just splits the userbase and makes each individual SKU less profitable.
I don't agree with your other point though, it imply that people that play Nintendo games don't play other games and vice versa.