Hello Kai
To answer your question, we need to first see what events will trigger a PIM Assert forwarder election. Take a look at the topology once again:
When R1 begins to send multicast traffic, it will go to both R2 and R3. They will both forward traffic to the network segment with the switch, and due to the nature of multicast traffic, R3 will “see” R2’s multicast traffic, and R2 will “see” R3’s multicast traffic. It is this detection of duplicate traffic that will trigger the election.
Now in the scenario you suggested in your post, we remove R4 (the receiver). If there is no R4 to request traffic then you have eliminated the conditions under which a PIM assert election will take place. You also eliminate the multicast traffic as well, so the issue of how pruning will take place is moot. Does that make sense?
I hope this has been helpful!
Laz