It does seem like a long time. The question for me is: if it is a port, why isn't it instead a new 2D Mario?
I'd think that the 2D Mario staff would have to be up to something else besides a port and if it were 2D Mario I don't think they'd need to do the port, which makes me think about either the new IP mantra or SMM2 taking priority over a new 2D Mario while Nintendo still has the good sense to get 2D Mario on the Switch in a timely manner.
Or perhaps it is not a port.
A port is much easier than a new game. It took 3 years to develop NSMBU according to the Iwata Asks. It's a very good game and deserves to be played by more people. There's no doubt on my mind we'll get a brand new 2D Mario at some point for the Switch.