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.
Handhelds are not an accurate comparison. Switch is not a handheld exclusively. No Nintendo home console has gone longer than 6 years. Nintendo won't risk releasing the successor later than 2023 and getting that behind in terms of what's being done in the industry. The Switch is going to start to feel archaic compared to what developers are doing on PS5/X4 by 2023, Nintendo will feel like they're in the dark ages if they push the Switch further than that and it will hurt their reputation.