No, they wouldn't. On Youtube, for example, videos were consistently trending longer over time, and you used to see frequent explainers (https://www.wired.com/story/youtube-video-extra-long/) on why this was happening and how Youtube benefits from it. Short-form videos are harder to monetize and reduce retention, but users demand them so strongly that most platforms have built a dedicated experience for them to compete with TikTok.

If that was true, I would be able to turn off shorts from my recommendation feed.

You can. It’s not a hermetic seal, I assume because they live in the same database as normal videos, but if you’re thinking of the separate “shorts” section there’s a triple dot option to turn it off.

I've clicked this triple dots many times. I never saw such an option. I saw "show fewer shorts", and even that seems to be temporary.