Would sitting on the bench at Bayern be such a bad thing? Definitely if he stays on the bench for a few years that wouldn’t be great, but would one season be so bad? He’s only 20 and he’d still be learning a hell of a lot. Might make a few substitute appearances in the bundesliga and maybe a few starts in the cups, which doesn’t sound like the end of the world to me.
It's by no means a bad thing, but it'd be far better for his development to play week in and week out - ideally in the same league as his parent team.
Also if the coach gets sacked half way through the season then he's in a much better position getting minutes in the Budesliga rather than an unknown (to the new coach) benchwarmer.