Okay, engineering time costs money. Absolutely staggering amounts of engineering time were spent ensuring that the extremely intricate, highly prone-to-failure deployment process would be successful. With a larger payload diameter, the mirror deployment process could have been dramatically simplified, thus saving some percentage of the staggering amounts of engineering time expended, thus saving some percentage of the billions of dollars spent to get the telescope to the launchpad. I do not care what company produces the launcher with the bigger payload diameter, OP specifically asked about Starship, and the answer is YES, a launcher with a significantly larger payload diameter than what is currently available, like Starship if it is ever successful, would make an undertaking like the JWST easier to design, and thus cheaper to design. I dislike Musk as much as any disinterested party (I'm sure people he's scammed with his "FSD next year" or people he's sexually harassed, and definitely his children dislike him more), but a better launch vehicle WILL make bleeding-edge space missions cheaper, and better. That is wholly unrelated to one obnoxious egomaniac.