Yeah I understand what you're saying...another part of the reason why John Logan's script was flawed. Shinzon should have continued his focus on the Star Empire, target more planets with the radiation forcing Starfleet to intervene to stop him. There's a lot of things in Nemesis that don't make sense I guess.
I never understood why the Writers wanted the Remans to destroy Earth or conquer the Federation. That makes them no different, as Villains anyway, then the regular Romulan hardliners who've wanted to destroy the Federation since the days of Captain Archer. Which, for an audience who has been following Star Trek for so long, is boring and, ahem, fatiguing.
A more interesting approach would have been to leave Earth out of it altogether and have Shinzon continue on his, and more importantly the Remans, rampage of vengeance against the Romulans. This would have given the actions of Picard and Co. more meaning to the Star Empire and a motive for establishing peace with the Federation. Kind of similar to how the actions of the Enterprise C help lead to an established alliance between the Klingons and Feds.
This would have also further developed the Nemesis theme by including an exploration of the differences between the Remans & Romulans, along with the juxtaposition of Picard/Shinzon and Data/B4.
Cool idea. Frankly I thought the mere concept of 'Remans' was bollocks. But if it had to be done, your way would be interesting.
Now, I want to make it very clear that I don't think STV is a good movie, but the plot is pretty basic and makes perfect sense. Spock's brother is getting visions of Sha Ka Ree and needs some way to get there. Since no one in their right mind would try to breach the barrier, he has to steal a starship. When he finds "God", it isn't what he thought it would be and then they kill God and live happily ever after.
Also worth noting that the novelization was GREAT.