Considering that the Switch in handheld drains the battery like crazy, I don't see it taking over for Nintendo's handheld side. The 3DS can get around seven to eight hours of battery life (at least mine), and the Switch can't come close to that.
Eventually the system will become obsolete, but that day is not today.
Yeah it look like it will since they made it so hybrid and you pretty much have to dock it to the battery stand thing 24*7. Which is kinda interesting they made it that way if they want it to be as available as the Switch. So unless they upgrade the battery and/or make the screen smaller etc. and lower the price it's probably gonna be like the Wii U thing in a few.