Wouldn't an app like this stop working after a few uses?

As I understand, the protocol penalizes users that don't contribute to the upstream, although I never checked the details.

Or do this kind of app keep changing the identity to avoid getting downgraded? Does Stremio work like this too?

Outside of a private tracker (which takes measures to keep random untracked peers from getting on the torrent), not really. Individual seeder clients can detect bad behavior like leeching and ban by IP, but each torrent is likely to have a different seeding pool.

So the penalty is mostly just on individual torrents. Of course, trying to pull something like this on a private tracker would get you banned real fast...

I was under the impression that part of what made bittorent work was that the protocol tried to estimate how much each peer is uploading moment to moment and only provide it that much data to download.

Oh definitely not, a limit that harsh would prevent most people from getting the whole file.

Uploaders get priority. But if you show up to a torrent past the initial ramp of growth there will be plenty of bandwidth to go around and you'll experience a high speed download regardless of your ratio.