It's pretty much etched in stone at this point that the Switch is Nintendo's next handheld, as well as their next home console. However the Switch isn't exactly being marketed as a successor to the 3DS, and Nintendo has vaguely promised some degree of "support" for the 3DS that will continue through this year. On top of that Nintendo's handheld market has existed separately from their home console market for the most part, with the former seeing more success than the latter.
I think Nintendo's ace-in-the-hole will be Pokemon. I think once we see Pokemon on the Switch, then most 3DS owners will get the message and take the plunge. However, I wonder if we're already seeing this migration now. Might that explain the Switch's massive success so far?