I don't think it is a simple reconessance plane (a spy) as this job can be fulfilled easlisy by satellites.
That was the reason given for the SR71's retirement. Well, that and the high cost of fielding an SR71. Personally though I find it hard to believe that there is no value in doing on-demand aireal photo-recon anymore. Satelites don't cover the entire earth at all times so if something is going on at a specific time and place that's not under a satelite and you want a photo then there is a need for a platform. It seems that this would be enough reason to keep at least one or two SR71s around "just in case" for quite some time since, though expensive, it was by all accounts effective.
SR71s overflew Libia after the first bombings to do BDA before the locals could cover up for satelite overflights. Wouldn't such capability still be valuable? What if there's another Cuban missile crisis type thing? U2 photos proved pivotal during that event.
So I think there's a definite need for the capability. They retired an aircraft which had the capability. That's a lot of fuel for speculation about how the job is currently being done.
As for what "it" is that's doing the job today I don't expect we'll find out until the platform is obsolete. Manned, unmanned, high speed, low speed, high altitude, flying in the weeds... some are more likely than others given our knowledge of the past platforms, technology, and challenges. But beyond that everything is speculation. Just like those old F19 photos and models. In the end they were nothing like the actual plane in appearance, function, or mission.