I firmly believe that by the time E3 2019 rolls around, Nintendo will come out and say the 3DS is being discontinued. Already there is enough evidence regarding Nintendo's behavior that points to them being ready to retire the console.
The first major point is the life span of Nintendo's other handhelds, which accounting for mid-generation refreshes, gives each console a 3-44 year lifespan. The DS was released in 2004, then the DSi in 2008, the 3DS in 2011, and the N3DS in 2015. By E3 2019 the New 3DS will be over 4 years old in all regions, making it a good time for a new handheld, or just discontinuing the line, which is the route i think they will take.
Not only that but Nintendo is demonstrating a lot of the same behaviors that they usually show when a console is nearing the end of its lifespan. They outright said that they have plans to continue supporting the 3DS is 2019 alongside the Switch, but this doesnt mean much since they say this all the time and never live up to it. They said the GBA would receive support after the DS was released, which it didnt. they also said the Wii U would still receive support after the Switch released, which it didnt. So why should now be any different?
Even considering how the audience reacts to 3DS game announcements is a big red flag for the console. People simply do not want to spend more money on 3DS games, myself included, and I know far too many people who feel the same. The switch has redefined handheld consoles, so seeing dedicated handheld games feels like a missed opportunity, like the games could have been better if they were made for the Switch. Even GameFreak is jumping ship by now making the Switch the new home for Pokemon game releases. so in the scenario where Nintendo doesnt abandon the 3DS and/or makes another handheld console, it will be Nintendo's first handheld EVER to not have a pokemon game. the games which move more software sales than any other on Nintendo's handhelds, would not be there.
so im firmly stapling my prediction in now. By 2019 Nintendo will formally announce the 3DS is discontinued