I'm fairly sure that long term TikTok plans to do long form videos too and their current approach has a benefit of getting their foot in the door. No other competitor of YouTube managed to do that before.
The neat thing about TikTok algo is that that they introduce different things and fine tune it to your liking without making things stale. This means those that are there for short form videos will keep on seeing them and their flow won't be interrupted. Those that are interested in longer ones will be presented with them. It's already happening but UI really needs refinement as it's kinda jarring now. It's still much better than how YouTube is trying to force shorts on their current users though.