Maybe, but again, I don't see this as an issue. Besides, the rogue has more going for it than sneak attack damage. The caster might not even be able to maintain shadow blade, especially if they have other concentration effects they want to keep up or even just other spells they need to cast. Rogue doesn't have this issue. If I had a rogue player who was complaining about it, I'd suggest they reroll as something else. More likely though, they aren't complaining because they wanted to play a rogue and they're having fun playing one.