The only way I see them releasing a successor to the 3DS or "portable" is creating a smaller Switch unit and call it something like the Switch-Mini that will be unveiled alongside the the Switch successor in 2 or 3 years (like the iPhone 7 to 7 plus or Galaxy s8 to s8+). That can easily fit in a pocket. Both will use the X2 (or X3 if Nvidia releases an X4 by then). I fully believe the days of the dual screen (aka DS-line) for Nintendo is numbered. Maybe it loses some of its 'special' features (removable joycons, etc.), or maybe they make the new mini-dock an optional accessory (as its supposed to be a portable device), or downgrade the display (720p screen while the Switch 2 gets 1080p) to get it at a 'cheap' pricepoint of $50-100 less than the regular Switch. My gf would definitely get a Switch-Mini, since she says she can't put the normal Switch in her pocket and is the reason she won't get the system even with a new Pokemon game (main/only reason she owns a 3DS).
Its easier and more efficient for Nintendo to have one platform and release all games compatible.