The story already had a number of elements that would have allowed Stewart to do part
of the Shinzon role - probably to the dramatic benefit of the whole.
The Romulan plot was that Shinzon was supposed to eventually become Picard. The Shinzon hope and fear was that he would become Picard; the Picard hope and fear was much the same. There was a lot of unused potential there...
What if Shinzon does undergo the rapid aging thing after all? What if the only person to witness this final development is Picard, possibly strugging with the villain aboard the Scimitar
as in the aired movie? And what if, after a struggle between two Stewarts, the winner is left ambiguous - and the only thing we know for sure is that the man left alive demonstrates his fundamental goodness, saves the day, and sits (back?) on the center chair of the E-E?
Considering who wrote the damn thing, the least I'd expect would be "triumph through death" somehow worked in there...