You'll be surprised. People post videos with walls of text or post a video of them narrating whatever they were planning to write.

For now yes, but I would say it's really meant as replacement for Instagram rather than Facebook. So videos only seems fine compared to a service that was primarily for pictures.

They can always expand, but I feel like these social media services usually have a hard time pivoting to a different medium after their initial success.

TikTok feels like YouTube. It has hundreds of millions of users, but only a small fraction will ever post anything. Unless the dynamics of TikTok radically change, I can’t see it being an Instagram replacement.