A successor doesn't need to retain Joy-Con functionality.
So no detatchable controllers? I think that would defeat the purpose of a cheaper model when it forces you to buy an additional controller to play the system docked. Also the Switch is in a different situation from the 3DS. It had a unwanted gimmick that forced Nintendo to massively cut the price early and take losses. They couldn't possibly price cut it again so instead the 2DS was made. The Switch doesn't need a 2DS, it's selling great as is and can have price cuts gradually. 3DS can fill the cheap handheld niche until the Switch's value gets there.