It would be really nice to be able to download all the games I purchased for the switch if backwards compatibility is in play. I don't necessarily think it's essential though, I plan on keeping the switches (plural) I've already bought as I like to do with all my old consoles.
What is essential is the ability to use old hardware with the new switch. At the very least, Bluetooth controllers should still be supported so I could use my switch pro and 8bitdo controllers. Joy cons and docks would be nice to have, but I think those would ultimately depend on what kind of form factor the new hardware has. I doubt Nintendo will abandon the hybrid dock able format, but we will see.