I agree that FS and US combined can be exceptionally powerful in the late game. They are a good way to exploit a huge advantage you already have in technology, infrastructure and resources. But look at the requirements needed to make that happen. You assumed 150 towns and markets, grocers and banks (total of 500 hammers) in each city. Say we have 15 cities with an average of 10 towns each, then that's 7500 hammers invested in infrastructure that the Nationhood option doesn't need. That many hammers can make a sizeable army before we start the comparison between the FS and Nationhood options. Quite apart from that, it takes most CE cities a long time to produce 500 hammers as they are generally low production before US is available and they don't whip much as their priority is working cottages. And they need quite a few other important buildings besides (library, university, forge, barracks and so on) So expect a big delay in getting ready to use US and FS effectively to generate hammers from gold.
Perhaps a weakness in your assumption was that you can make war with US and FS. Late game wars are bloody and WW escalates at absurd rates. After a few big battles you would be forced to adopt Police State or to make peace. Is the combination of PS and FS as strong as PS and Nationhood in war - even in the very late game? I suppose FS could be useful to provide gold to upgrade older troops but that is expensive (3 gold per hammer plus 20 gold) and they lose experience (over 10 exp). So it would depend on the size of your army and if you had many obsolete troops left you wanted to upgrade.
Summary: In the run up to war and if you had already invested heavily in infrastructure in your cities then yes FS with US is better than Nationhood. But if you want to prepare for war earlier in the game (without the infrastructure) the efficiency of US and FS is severely curtailed. If you want to have a long running intensive war you will have to lose the US civic and FS loses its major attraction. With those qualifications I agree with you