Update: Eugene Volokh has some interesting thoughts on where this all may lead, based on his work on the dynamics of slippery slopes in law and policy.

...the slippery slope may well be in operation here -- I'd call it a mix of the simple attitude-altering slippery slope, either an erroneous evaluation slippery slope or an accurate evaluation slippery slope (depending on whether you think the public will correctly estimate the value of the project), and cost-lowering slippery slope (in the sense that providing the government with more experience about how to run TIA-type projects will lower the cost of broadening them in the future). But some slippery slopes are good, if you already think that what's at the bottom of the slope (e.g., the broader TIA) is good, or if you're not sure whether it's good but you think the first step might indeed provide useful information about the likely value of the next step, and you think that the political system will act soundly based on that information. And of course it's possible that the slippage won't occur, because the public will maintain the line between the government simply organizing the data that it already has, and the government also incorporating data from private sources.
I think he's right -- these initial steps for TIA are designed to make the next ones easier. Starting TIA with a small data set is not intended to limit the project's eventual scope. Rather, it's designed to assuage concerns today about TIA, so that people will object less in the future to the project when it does acquire more data. If you object to TIA in its current prototype form, with its present data set, the time to speak up is now. If TIA proves itself with the government-owned data set, it will most likely be expanded to include medical information, credit information, and other indicators which may be valuable in the national security context. The slope becomes both steep and slippery from here.