This will be a very risky move from Nintendo if they really decide to left the home console business with NX. Most western 3rd parties nowadays dedicate their efforts mostly on competitive, high-end hardware. They have been ignoring the portable market since the late DS days mostly, despite the high userbase (and potential sales) from systems like DS, PSP and 3DS. If it is a portable that you can play on TV, it'll hardly manage to attract the home console audience. Home console players like to high-end, high-performance graphics on the TV. This is the standard set by PS4/XBO's market. Nintendo will lag behind them if they bet on underwhelming hardware, whatever the gimmick or unique gameplay idea they introduce. This happened with Wii (in it's late life mostly when the hardware difference between it and the HD twins became clear) and Wii U. It won't work again. This is painful for some, but the gaming market prefers hardware performance over unique and innovating gameplay. Wii's unfortunate premature death is clear. Nintendo also tried to rely on it's GBA's library to boost GCN's sales with the Game Boy Player. It didn't work. Portable players don't want to play portable games on the TV and home console players doesn't care about it, either. Only a niche market will want to play portable games on the TV.
"Why bank on a successor of Wii U?" Because home console gaming is where it lies most of the dedicated gaming audience and where most gaming devs dedicate their efforts. Mobile might be growing, but the center of gaming industry is home consoles. Nintendo abdicating this market will forfeit a market they're huge responsibles for exist in the first place, as they still have a strong name and presence, despite the recent Wii U failure. It was their lame hardware decisions, beting on unique gaming at expense of hardware performance under the creed they can't compete against Sony and MS (which is bullshit, because Nintendo manage to turn around against Genesis, was selling almost head on against PSX in it's initial years and owned the competition in Wii's early years), that repelled the market and support away from it.
Also, becoming portable only won't make Nintendo exempt of competition, either. They'll face mobile competition, a market Nintendo itself is supporting. They're aware the casual market they had with Wii and tried to attract with Wii U migrated to mobiles, hence their decision to go there. Whatever the way they choose, they can't escape competition. If they can't afford (or don't want) to compete, they should leave the hardware market and become a software only dev. I think your vision supports Nintendo's defeatism.
Make sure that Nintendo is far, far more remembered for games like Super Mario Bros., The Legend of Zelda, Metroid, Donkey Kong and Pokémon than Brain Age and Nintendogs. It's identity was built thanks to them. Yes, games like Wii Sports, Wii Fit, Brain Age and Nintendogs might have sold better than all of them, but they underperformed on 3DS and Wii U. Sales not always translate into legacy.
