The power of reps extends far beyond just passing legislation, and even then, essentially all of them are on some committees. Lots of them (maybe even most/all?) make themselves accessible to constituents to help work through government bureaucracy, and a representative shining a light on a constituent's issue can help things get addressed. I'm not sure jumping straight to 10,000 reps would necessarily be the best idea, but I think having been static for the past 100 years isn't ideal either.