I agree, as well with things like sauces from shellfish shells that would otherwise be tossed.

However, I recently heard the reasoning (of a buddhist who tries to avoid animal products, i.e. would not actively seek out a non vegan option, while accepting non vegan food if offered) - would the product in question still exist if the animal was seized to be killed? Even if it was for a different purpose and leather, seafish shells, gelatin etc were just byproducts, they would not exist if the animal it is derived from wasn’t killed for whatever reason.

I totally agree that it would be a shame to only use a part of an animal and let the other go to waste. Likewise, I get that “I’ll do X as long as everyone else does Y” is a way of reasoning that ends up counterproductive and keeps us locked in a status quo. So, if someone wants to stop using not just the primary products of the meat industry but also the secondary, I can understand.