It seems obvious that talk of a 3DS being supported past-2018 is possible but also just as possible a 'third pillar' talking-point that is being stuck-to coming off the back of the surprising Switch popularity that still needs to fully embed (we are only 3 months in, after all).
It also seems obvious that with the one dedicated platform of Switch, Nintendo is now able to supply it with titles at a rate of 1 game to every 1 or 1.5 months. Next, when looking at games announced and schedules, we can see that these release slots have already been filled up to the end of Nov/Dec (with Xenoblade 2) - Yoshi/Kirby will most likely be early 2018 releases and were announced now not to take away any hype from Smash, which I fully expect to be announced in an August/September/October Switch Direct, focusing on 2018 titles and beyond.
Smash is coming, and I would expect its release before the end of April 2018. Willing to bet a tag on that easily.
-STEALTH EDIT-
To back up, my thoughts on the release dates are;
June: ARMS (Now)
July: Splatoon 2 (July 21st)
August: Rabbids x Mario: Kingdom Battle (August 29th)
September: Pokkén DX (September 22nd)
October: Fire Emblem Warriors ('Fall' - likely before Super Mario Odyssey)
October: Super Mario Odyssey (October 27th)
November: Xenoblade Chronicles 2 ('Fall' Likely late-November, very early December)
-2018-
Jan-Feb-Mar-April: 4 months = Yoshi Switch, Smash Switch, Kirby Switch, NHM3/SMT (can't decide, possibly neither)
May-Jun-Jul-Aug: 4 months = Unannounced Titles, Summer mainline Pokémon Title (could be Sept.)
Sept-Oct-Nov-Dec: 4 months = Unannounced Titles, Fire Emblem Switch (again, could be Oct), Metroid 4 at Christmas, possibly delayed to Feb 2019.
The above doesn't seem wildly out of the realms of possibility.