copying a know feature by a company that's bazzilion times bigger is not "competition". it's rather a lack of regulatory oversight that ket crappy FB/meta siphon all the competition... and at this point whatever they can't buy / kill they just copy and extinguish...
Copying features is
often competition. A disappearing photo is not a unique invention. The concept is simple, the photo disappears after being viewed. What matters is the implementation, the interface, the surrounding experience, and whether users prefer using it there.
Instagram adding disappearing photos is a normal competitive response. Snapchat made a feature people liked, Instagram saw demand for it, and users get a similar option in an app where many of their friends already are.
The size argument is weak here. Snapchat is not a small indie app getting crushed with no way to respond. Snap Inc is a multibillion-dollar company with a popular product, and it has made choices many users dislike, encrapifying. If they continue making their product worse with ads, clutter, and unnecessary features, users leaving for a cleaner experience is the market responding.
By your logic, nobody should open a clothing store because Macy’s has more money and brand recognition. Bigger companies having advantages does not mean competitors get to own a category forever. Unless Instagram is infringing a patent, blocking Snapchat from competing, or using exclusionary tactics, offering a similar feature with a better user experience is competing.