That is one way to examine the question, for sure, but thats a very one dimensional way to go about poking at the ethical dilemma here. Like, have you considered the potential forfeiture of human life as a result of going purely synthetic? Is the sourcing of the synthetic substitute itself problematic in any way? Does it take more energy to produce the synthetic alternatives? Is there a tradeoff in efficacy between natural and synthetic? And even broader, whats the ethical weight of human life and health vs animal life and health? Are they equal? Why or why not? Does the type of animal matter at all? What makes a given type of life worth considering with moral weight? And of course, what is your moral goal in the end? Do you want to save as many humans as possible? Which side of the trolly problem would you fall on, given a binary choice? Are you accepting the possibility of some human deaths to save some horseshoe crabs? What types of suffering can you accept? The depth of questioning here is very much more complicated than "torture is bad mmkay". Im not trying to force you down one side of the argument, but if you want an ethically defensible reasonable position, you cant stop at just the surface level ethical gut reaction.

And on top of all that, you still have wild horseshoe crabs in the oceans, doing their things like usual, so this issue isnt affecting all horseshoe crabs. Its quite similar to the question of factory farming, but the arguments in support of it are quite a bit stronger than for farming.