You miss the point here. Even the 3rd party games are published by Nintendo with the exception of I think 3 or 4.Yes I got your point, that's why I said that the top sellers being Nintendo games doesn't mean anything for third party games. I thought you were talking about sales and saying it was lack of third party games that made the Witcher do well so apologies for that misunderstanding.
I class March 2017- February 2018 as the first year, so the fifth year will be March 2021 to February 2022. Based on the known line-up I'm expecting it to be the best third party year both quality wise and sales wise. Nintendo has said the Switch has just entered the middle of its life so I'm not expecting a successor until holiday 2023 at the earliest. Why do you think it will come earlier?
Simply due to tech availability and Nintendo's general release schedule. They usually go with a 5 year cycle and unless they truly are going to milk the Switch dry I'd expect something in 2022.