The Wii U was a commercial failure and less powerful than the 7th gen's Wii competitors, so Nintendo really needed a new console and to escape the Wii brand, which is why the Switch came out less than 5 years later. The Switch, however, while less powerful than its competition, is Nintendo's most commercially successful console to date excluding handhelds and still has a lot of momentum.
The Switch's main problem is its lack of power, which will inevitably affect its third party support in the not too distant future. Nintendo's first party software is far more successful and profitable than third party games though, so that doesn't really matter.
Point is, the Switch isn't a commercial failure like the Wii U was and it continues to sell well, likely outselling the Wii within the next year, so it makes sense for Nintendo to continue capitalizing on its success for as long as they can.
I think at the most what they might do is give us a tease next year like they did with the "NX" in 2015 with a release of a brand new console no earlier than 2024. The Switch is still going strong so there isn't much reason to release a successor yet.