Many have doubted Elon Musk during the early days of Tesla and SpaceX thinking he was incompetent in running those companies and the goals were lofty. People still doubt him with as much ferocity as his fans that adore him. It’s super fascinating IMO.
That being said, I do think Instagram will have some success with threads the same way Reels has been successful in fending off TikTok (as in not made completely irrelevant). People who share on TikTok also cross post on IG reels for more views and for eyeballs that are not on TikTok. I think the same thing will happen, where there will be some crossposting. Twitter still has a large audience - that will still make it relevant for some time.