Lets face it, they would never do this tough. Why give people an option to play their DS/3DS games for free when they can charge 10-20 bucks for them on the Eshop?
I guess I am seeing it as the 3DS hardware has become ancient now and sliding in a 3DS slot into the Switch means Nintendo can stop production on the 3DS (If they haven't already) and drive the 3DS market to the Switch. Eshop would probably counter this however 3DS games cost more than $20 so they can still physically sell $40 3DS games for Switch users instead of going the Eshop route or buying those games on ancient hardware that is separate to there mainline machine.
I agree that Nintendo wont likely do it but it would be cool.