Nintendo will probably release an upgraded version of a 3DS like machine in 2018, they already hinted the 3DS line was also their focus.

Switch will become like an higher end hybrid while the 3DS line will remain the lower tier cheaper machine with size and portability in mind.

Probably from 2018 onward most games will be released for both, in low res on the DS line and HD in the Switch, which is rather easy, if they implement a cheaper ARM SoC on a future low tier console (probably some lower Tegra SoC also).

Is monster hunter still exclusiv for nintendo or could we see the next entry on other consoles too?

Probably not, the series is well estabilished on Nintendo and their userbase is very faithfull. I could bet money on the fact that this series will remain a Nintendo exclusive, at least this new (Switch) generation.

Just a slight correction. 
MonHUn was also established on Sony's sysyem but that didn't stop Capcom from releasing MonHun to the 3DS.
The userbase is not faithful to the platform and will go on what platform it was released on. 

True enough.

IMO, it's a money question though.  The Vita is in a pitiful state overall, they aren't going to put it there.  And the 3DS is looking to fade as well.  Capcom has shown a propensity for skimping on certain costs when it comes to MonHun so I fully anticipate  them going for the platform where reaching "acceptable" visuals is cheapest and that's the Switch hands down, no question.  Plus the portable MonHun scene is really big and with then 3DS already setup to fade away, the Switch will be the only remaining healthy portable ecosystem.