It was inevitable from a game development POV, once portable graphics go past rudimentary N64 to Dreamcast tier visuals, the amount of resources the portable line would require is basically the same as a console.
And no company can support basically two console lines. And you saw that with Wii U and 3DS, Nintendo had massive problems getting games out for both at a regular rate. Ditto for Sony with Vita and PS4.
And "3DS-2" would've presumably been another generational leap making things essentially impossible.