Now there is excellent I2P support. However on I2P there are many fake leechers, fake seeders, test or development or simply non-working installs of torrent clients, that never complete a single piece. To make "Last Activity" meaningful again, I suggest advancing that timestamp only in two cases:
- we complete a fresh piece
- a leecher sends a "have"-message for a piece not owned before (regardless where that piece originated)
Everything else is highly likely to be a non-event.