I've always hated how discussions on workers' rights always seem to revolve around what people 'deserve', 'fairness', 'disrespect/devaluation', 'robbery' and all that moralistic stuff. There are differences between different industries, you're going to have to end up comparing them, and bringing in all this moralistic language inevitably makes any discussing about differences/imbalances sound like 'X is better than Y'. And we wonder why no-one can reach an agreement 
Just talk about it terms of cold, hard logistics; 'is it worth it for the worker, yes/no'?
For instance an artist, after making an estimate of how much money the comic is likely to make, decides it's only worth their while if they get 70% of the revenue. This isn't a matter of 'artists are better than writers'; they're happy for the writer to have 70% of the revenue as well if it weren't logically impossible because by definition, you're not going to be earning 140% of the revenue for your work. There's only 100% to go around.
It's not a matter of "I'm doing more work than the writer, so I should earn more"; otherwise they'll be fine with them getting 7%, the author getting 3%, and donating the rest to charity or smth. It's a matter of "it's only worth it for me if I get 70%, and unfortunately because of the way reality works which I can't control, this leaves the writer with only 30%".
And maybe it just so happens that more writers are willing to do it for 50% while fewer artists are willing to do it for 50%. I'd love it if it were viewed as a statement on what's a good deal'tradeoff for them, and not "what they think they're worth". 