Currently, max-concurrent-downloads setting counts all downloads, including those seeding-only torrents. This actually prevent to let aria2 be a long-term all-in-one download manager, including to be a bt (especially PT client). It would be much better count those torrents separately. I.e., there should be a setting like bt-max-upload-slots. And if compatibility with old behavior is important, the old behavior can be restored when bt-max-upload-slots is set to 0 or -1 (depending on which means unlimited upload slots).