ovovo

For the tracker itself, because it is much easier for the police to go after trackers that server thousands of pirates than to go after the individual pirates.

To be fair, I thought of modifying my client privately to do it, but I guess it will only have an impactful network effect if it is distributed as native feature of popular clients.

Yes, with this solution, internally it could be seen a two separate torrents, but if it is an option easily accessible in the client settings, and it is handled transparently as a single torrent, much more people will do it, and the scene as whole would gain with the network effect.

Your suggestion would purposely share the same exact private torrent infohash into public DHT/PEX,

Yes, but not necessarily. It is trivial to recompute the infohash with the private bit disabled. This would split the network, but that is probably a good thing to preserve the anonymity of the private tracker users, as pointed out by another commenter.

I thought that it was so it could maintain a lower profile, thus attract less unwanted attention, and maintain the health of the torrents with the minimum ratio rules.

But I am not dismissing this issue, I think it is important.

But if this is a concern, the swarm itself could be split into internal/external, and no PEX would be allowed to happen for peers that are received exclusively from the tracker. This way, peers who have the setting enable would act as bridge between the two swarms, and only their IP would be visible.

I don't think so, otherwise magnetic links wouldn't work.