Without backward compatibility, I will have to pay 300$ for a console that have 2-3 games and when I done with them it become 300$ paperweight.
Backward comaptibility was one of the thing that make people decided to buy a console at launch, because it give them something to play while waiting for other title to come out. Also it help low the cost for new console by sell the old one, plus no one keep more than one console at home except real hardcore gamer.
So all in all, it a bad idea to buy Switch at launch without any solid backward compatibility.