Nintendo already has 1st party titles announced for 3DS in 2019, how are they going to expect to sell Mario & Luigi: Bowser’s Inside Story + Bowser Jr.’s Journey if they don't even make or sell the system anymore.
2020 seems more likely in my opinion, they have 3DS already foretasted to sell a certain target until March 31st 2019 and I don't seem them discontinuing it just 2 months later in June 2019 when Nintendo themselves have promised new 3DS games will be coming "in 2019 and beyond". http://www.ign.com/articles/2018/04/27/nintendo-promises-support-for-the-3ds-through-2019-and-beyond