Everyone I know that uses Snapchat has migrated to TikTok, but I personally have limited insight into these dynamics. From my perspective, even though the sharing dynamics are different, TikTok seems to capture and hold much greater attention.
With TikTok, you're publishing to the world; with Snapchat you're publishing to your world.
Private secondary Instagram accounts (finstas) for smaller circles and Instagram's "share only to 'close friends'" features are taking a lot of that Snapchat market over.
TikTok, Instagram, and Facebook are all places where people share more public / curated content, whether that's a silly video you made, a meme, or a little more curated photo of something exciting you're doing. Regardless of the content, there's a big focus on reputation and superficial interactions (likes, comments, etc.)
In contrast, Snapchat is still extremely popular for private communication (group chats, private messaging). For many college-age people, Snapchat is closer to an alternative to text messaging -- people heavily use the editing and multimedia features in creative ways that you couldn't do with normal SMS (For example, drawing over an image, circling something, overlaying another image, typing out a sentence over an image). Snapchat still has a consistent user base here because they've continually focused on improving the chat features. One example of this phenomenon: Whenever I met new people in college, we exchanged 1) Phone numbers 2) Snapchat accounts, since those are the primary methods for private communication for many. My snapchat friends list is nearly 1:1 with my phone contact book for anyone I met socially.
Instagram competes with Snapchat (and is arguably winning) on any feature that focuses on public sharing, such as the "stories" feature -- And for good reason, since Instagram as a platform has been designed from the start around public sharing. Similarly, TikTok will definitely beat Snapchat on the curated public video content, because that's just never what Snapchat was good at.
Only time will tell whether the smaller user base that is hyper-loyal to Snapchat for private messaging is large enough to justify their insane valuation, but I don't think Instagram or TikTok have really captured Snapchat's core value in private messaging.