It wouldn't really affect my purchase decision. Lack of access to older games is my biggest criticism of Nintendo (their disgusting practice of releasing older titles exclusively via subscription on Switch makes it even worse!), but Nintendo and their games aren't must buys for me. I wouldn't get a Switch 2 until a number of interesting games are available and the price is significantly lower. Backwards compatibility isn't going to affect that since I already have a Switch, I'm still able to play those games.
I suppose it would be a factor if my Switch were to break, then the choice between buying a replacement Switch and Switch 2 would be heavily impacted by the availability of backwards compatibility. But that's the only scenario I see it making a difference.