It likely wouldn't sell well enough to warrant developing not only hardware that could handle it without killing the battery life (the 3DS did it with two superimposed screens, and a second screen is energetically expensive to run), but also the appropriate firmware-side solutions for applying the effect retroactively to games that weren't designed around it. Even then, applying it retroactively like that probably wouldn't look too great, so you'd just be left with a handful of new titles that would actually try to take advantage of the feature, but that's still not overly worthwhile because the Switch 3D's install base probably wouldn't be large enough to justify it. Toss in that the feature would be utterly useless for anyone that only plays docked, and it's just not a worthwhile endeavour.
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