The only way they can replace the 3DS is if they make the Switch just as easily portable, and from what we've seen so far, it isn't. So no. Phasing it out is stupid until the time when the Switch is just as portable.
Sm4sh was like what you want. The Wii U version was irrefutably superior and had a lot more features such as 8 Player Smash. Maybe FE Warriors will too. And even if not, that just means 3DS players aren't getting screwed over and there's nothing wrong with that imo.
A handheld replacing a handheld is not unusual. There's no reason to phase out the 3DS because it's far more easier for it to be portable. The Switch is too big to fit in pockets and we don't even know if it has a sleep mode or not. It won't be easy to bring it everywhere or stop and start...