It seems rival agencies (Chinese, Russian) should be interested in doing the same, or at least denying NSA this capability. I mean adding some exit nodes is not exactly expensive, seems like a low hanging fruit, doesn’t it?
Yeah, that's another argument. The NSA competes with its counterparts to own Tor infrastructure. And that competition prevents any one from owning enough to pwn users.
And it's no accident. Tor was designed that way.