They are still looking for the key art director. They are clearly only in early pre-development phase, and actual development will take 2 full years at the very shortest. There's no way this is releasing before another 3 years, that would be a "normal" development time for a Nintendo project, not a long one. And 3 more years would also be a normal 5 year system cycle for Switch. Add another year because of the Switch's success and we would still almost surely see the successor in 2023. Nintendo will be well aware of this, hence why I think they would have the foresight to want the project moved to the successor.
These days there are multiple lead artists, lead programmers and the like working on big titles. We got a report that Retro Studios had put together a convincing demo to get the job assigned by Nintendo, so I very much doubt that the game is in a state where work has barely started because apparently Retro already knew what they wanted to go for last year. (The announcement of restarted development was made in early January.)
Five years is not a normal lifecycle for a successful Nintendo console, that would be six years or more. Most recently, the DS went six years and three months, the Wii went six years and the 3DS went six years. Six years is not the ceiling like you suggest, it's the floor. If Switch lasts as long as the 3DS, it will be replaced in March 2023. But the thing is that Switch is much more successful and healthier than the 3DS, so it would only make sense if Nintendo took advantage of the fact that over two years in they didn't have to cut the price of Switch nor did they release a revision yet. Switch can easily go beyond 2023 until a successor is necessary.