I see similar arguments to this one a lot and frankly I don't see how anyone with an active social life can believe it to be true. The userbase for these platforms has massive crossover. Maybe it's a generational thing or something but I can gurantee you the socially active Gen Zers are on Insta and TikTok. If anything they'll be on both of those as well as Snapchat and Discord using each for particular types of social interaction. So the issue clearly isn't a locked in userbase.
So it must be the content creators right? But this is rediculous too! Because virtually every content creator worth a damn on TikTok will have a linked Instagram! What's more, many of them will often have a Linktree or similar url either in their profile description or linked on their insta that will include everything from YouTube, to Patreon, to OnlyFans. People who understand how to make money making content on these platforms aren't one trick ponies, they're using sophisticated marketing approaches that leverage the strengths of different online platforms to compensate for the fact they often have handle their own marketing.
So if the content of the knockoffs is bad or the creators arent finding the competition worth considering then the only question remaining I would argue is whether the problem is in the reccomendation system (TikTok was, and still is, famous with users for doing a solid job at this), the moderation system (TikTok, maybe because it's Chinese, is not so fixated on implementing the SJ nonsense that YouTube, FB, etc. pander to at the moderation level), or the compensation system (Again, probably because they care less about demonetizing you for wrongthink against whatever miniority or "important cause" is the pet of the week. Content competes primarily on its merits as likeable content).
In short, TikTok might have had an early mover advantage but the reason no one catches up in the current environment is because they can't get their heads out of the sand enough to realize it's their own fault no one entertaining wants to produce anything but TikTok reposts for them.