>>376585293DS wasn't a mistake. Most developers make do with the low system specs and small screen resolution, and build wonderful or at least stylistically fun/interesting looking games.
Gamefreak jumped the gun into 3D and future-proofed the series by making high-quality 3D models that they jam-packed into that little system. Doesn't help at all that nu-Gamefreak sucks at optimization and coding, making their games look and run like shit.
3D was definitely the next step but Gamefreak bit off more than they could chew, and it shows.
Only now will the future-proofing help them, going into developing for the Switch. Gamefreak has no more excuses to make crappy-looking/running games on outdated hardware.
