Everyone can throw around all conjecture they want, the simple truth is it's all up to the sales of the switch, everyone brings up "The DS will not replace the GBA" but at the time they honestly meant that, but the DS sold so much better than the GBA and had such a huge market that they would have only lost money if they continued supporting the GBA.
While i personally doubt that the switch will do that well (Especially for those too young to remember how hype the ds was, everyone and their grandma had one for everything from gaming, to recipes, to language learning, and if you had an r4 you had the coolest mp3player/ebook reader on the block) but if the switch sells so well that nintendo can't make a profit supporting the 3ds, then they'll discontinue it, if it sells allright but not as good as the 3ds they'll keep supporting the 3ds until they make a proper replacement or they feel it's not performing well enough. It's all about the money, the money maker gets the attention.
and if you are worried about the switch replacing the 3ds as the main handheld just remember, it's not the main nintendo handheld 'till it gets a main series pokémon game", if the switch gets gen 8 that's when it's all over for the ds line