First of all, no agency or government entity has unlimited resources. It’s a big part of the reason why, for example, the US sphere of military influence in Asia is waning. Every empire eventually stretches itself too thin.
What you’re proposing/questioning could either be called a 51% attack or Sybil attack. Once again, so much has been written on these topics (as well as node aging and management) that I implore you to use this forum’s search function to do more research. Here’s another useful thread for you to read: Step-by-step: the road to Fleming, 3: Sybil resilience simulations.
There is no way for any entity other than the Network itself to know how many nodes are on the Network, and the number of nodes isn’t fixed. Why does it matter that there are 5000 nodes to start? Moreover, what do you think these malicious nodes would do? If the malicious nodes are filled with junk and can’t store more data (would be) users request to upload, that would drive up the value of Safecoin, which would entice more farmers to join the Network until the laws supply and demand restored balance.
I suppose theoretically, a government could choose to keep trying to keep pace with Network growth by adding more compromised notes but, once again, that would be incredibly expensive both in terms of fiat spent and human capital deployed. Plus there’s no way for said government, as already noted, to be sure if they have x% of nodes (which would still need to age). I also think you’re taking for granted how difficult it is to generate the coordinated and collective action within a government for something like this. Every dollar spent waging war against SAFE would be a dollar many other parties would have wanted for their pet project.
Also, the Network would be aware, I would think, that a bunch of Nodes have many PUTS but almost no GET requests. I’d assume that would be earmarked as weird behavior.