Let's read your list and consider which of those options hosts long form video content
Reddit: a link aggregator, not a video hosting site Tiktok: Chinese data collection machine that hosts very short form videos Instagram: data collection machine that hosts pictures your mom likes and short form videos Twitch: focused on streaming and gaming, not a YouTube competitor
Reddit has supported direct video uploads for literal years, maybe 5 years at this point, and up to 15 minutes in length. TikTok also collecting your data doesn't change that it hosts videos, and supports up to 10 minutes, and that limit has been increasing as they try to be a more general purpose video platform. Instagram hosts pictures, but also livestreams and videos, both short-form (a few minutes), and long-form (IGTV supports to an hour). Twitch is the only one that seems to clearly not support long-form video content, but even then it's still a competitor because that's not all Youtube does either.
If more people get their video entertainment from TikTok, then they watch YouTube less.
Most blatantly, YouTube literally launched shorts recently - an exact TikTok competitor clone.