Thematically it would've worked better had Picard been the one to die.
1. It would've worked better with the story they were going for. King Arthur meets his long lost son Mordred. In the story Mordred strikes down Arthur as Arthur pierces him with a lance. Displayed in how Picard ends Shinzon.
2. Also had Picard died then Riker would likely been promoted to captain of the Enterprise. The rest of the crew would've stayed together after that. Worf and Geordi never left the Ent-E, Troi now Riker's wife wouldn't leave for the Titan, Crusher may have stayed on to keep the family together and Data would be first officer.
3. Riker's destiny would be fufilled by becoming captain of the Enterprise. There mission would be working on relations with the Romulans just like at the end of NEM. Mirroring Undiscovered Country by making a long time enemy an ally.
I really like how you've laid this out. It makes a lot of sense and would've made for a much better film. I like the description of Shinzon as Mordred quite a bit. And the way he did die could be seen as Arthurian in retrospect.