Most likely Switch sales will actually increase. 

Much ado is made about console power, but to the average consumer price matters more.  When the PS5/Nextbox is released customers will see a $200-$300 Switch next to a $400-$500 PS5/Nextbox.  Most are just going to choose the cheaper console, especially if it has a lot of games which the Switch does.

Probably nothing bad, other that it might be second or third on the charts

The switch will face a natural decline like every other console, but it has nothing to do with the next gen systems 

Nothing, Switch didn't hurt PS4/XBO sales and PS5/XB4 won't hurt Switch sales. It's different enough that it doesn't directly compete with PS/XB consoles and instead they complement each other.

Assuming PS5/XB4 release during holiday 2020, Switch will be about to enter year 5 so it will likely begin to stagnate or decline around that time regardless of new consoles.

Fairly crazy numbers when you think that Both Nintendo's machines combined is not too far off the combined sales of the other platforms tracked Switch/3DS (312k) PS4/X1/Vita (330k) If the X1 had one of its 70k weeks instead of this anomalous 95k then Nintendo would have lodged more controllers in hands than the others combined.

I think it might have exclusives maybe that will continue to push the sales, along with I know this is crazy but perhaps a price cut or two to keep the momentum going. There's also a chance than PS5 and Xbox 2 won't be portable so maybe that's a selling point of the Switch. 

I doubt it.  Days Gone will do well, but the people who would buy it likely already have a PS4 at this point.

Switch on cruise control

Yeah, i don't think it will move more units than KH3 + RE2 effect.

Well they are definitely not launching this year, so 2019 is a lock for Switch WW I believe.

Seeing how they will most likely come holiday 2020, most of next year is safe for Switch too as the sales for PS4 and X1 will slow even more.  Once they launch however, it will be the baseline that is important.

I predict the PS5 to sell similarly to Switch/PS4 when the launch rush clears if Sony does not drop the ball.  Switch will continue its current momentum due to the portability factor for another 2 years through 1st and 3rd party executives based on the architecture of the Switch, which is when the Switch 2 drops and closes the power gap to at least where it is now again.  A timely price cut will play into this, as the Switch will be much cheaper than the other two newer consoles, and 3rd parties will not be able to ignore the huge install base.  While multiplats will be more scarce, we will likely see more Octopath Traveler or Bayonetta type deals going down when it comes to 3rd parties at this time.

The OG Switch will become the new lite console for a few more years as bigger projects shift to Switch 2 moving forward.  This will allow for Nintendo's typical 5 year cycle and a 7 yr or more lifespan for the base Switch.  Rinse and repeat.

The NextBox will do better than the X1, but not as well as the Switch 2 or PS5 in the grand scheme and the base Switch will settle in around 100 mil WW sales.

That is my prediction.