Widespread pre-emptive testing was not what Theranos tried. Defrauding investors that the technology existed for widespread, inexpensive pre-emptive testing even at small scale is what Theranos tried.

There is almost certainly gold in them hills if you dig deep enough and survey systematically enough, but Theranos started with precise coordinates, claiming they'd "found it", and demanded investors for a mine, while privately they were thinking "If we don't find it on the surface, who cares, we got paid, this is how VC works".

Theranos’ specific claim was that they could do existing tests with way less blood and way less cost. One of the ways they kept their fraud under wraps was to simply do the tests the old way behind the scenes.

So, blood tests in general were not the controversial part of Theranos.

As others have pointed out, the obstacles to large scale prophylactic blood testing are false positives, and general resistance of health insurers to fund anything not strictly reacting to disease or injury.

Theranos’s issue was that their tests simply didn’t work. It was more “move fast and pretend to test blood”.