I thought a lot before posting it, but I realized that if I write it in a clear and respectful way nobody needs to get offended. 

Nintendo doesn't like to release new systems. They only do that when they are almost forced to do so. In NES era if it wasn`t for Genesis/MegaDrive competition they would have still supported only NES for a much longer time. They tried to use FX chips to postpone the release of new systems as long as possible, and if there were no PS1 they probably would just call it a day using chips in cartridges to enhance performance.

To show that, we have the Game Boy market case to show it to us, with no competition in this market the first game boy lasted almost 9 years, and 12 if we consider Game Boy Color a hardware revision instead of a new system. Nintendo DS lasted 7 years.

They never were very pro-active when developing new generations consoles when they are top dogs. And this posture is very different from Sony for example that always pushed the new generations even when they were dominant.

Why i`m saying this? Because people say Nintendo is innovative, and they are, but sometimes, like with Wii U and Switch, it feels like they look like they are always very reactive with the new systems, instead of being pro-active. But you can argue "Hey, but Nintendo don`t follow this trend of always more power"and exactly because of that I think they are reactive, if Wii sales were good they wouldn`t have come with Wii U, and if Wii U was a success there would be no Switch right now. They always wait to their sales to decline to ridiculous levels to release a new system, while Sony never feared releasing a new system in the peak of their previous one, like they did with PS2 (ps1 peaked in ps2 year), PS3 ( ps2 peak) and PS4 ( ps3 were slowing down but was far from bottom).

Yeah, that's how it works. No reason to release a new system if theres no competition...

Well, whether Nintendo liked it or not, they start the development of a new system right after the old one is on the market. And it's released when it's released, this has more to do with the schedule than what's happening in the market. Wii was killed because the shift of focus to next generation. Games take years to finish, so whether the console is still selling or not, it should hit the market when the games arrive, whether first or third party.

One of the reasons to release a new system is piracy and the old market being already flooded. Even if PS2 was released on PSX peak year, the piracy was rampant on the more popular games, and when you had these no-name games, they were sold for two euros on a bargain bin. New console is always a new market and a new box of DRM.

R&D is expensive, if you don't need to make a new console, you also can use the same technology with creating new games instead of constantly having to upgrade