Leaving this here as unbounded memory consumption growth is obviously unacceptable in many contexts and that issue remains. (TL;DR it matters if you have millions of unique ephemeral streams in your system that are all fed into a long-running Sink - in many cases you'll have natural process recycling and/or may be able to filter events prior to feeding them into the projector in the first place)

It may be that a more drastic solution (drop all state and do a more complete reset etc) is justified - a purge in many contexts might reasonably happen in intervals measured in hours or days.