Many of these early cancers are asymptomatic and universal diagnostic imaging in the absence of symptoms is cost prohibitive. Better blood tests address that and other important gaps.
You’d follow an asymptomatic false positive blood test with at worst a whole body MRI and at best a targeted imaging study (if the blood markers are specific). You’d have a further false positive rate from that but it drops off quickly. The cost is manageable. And some of that cost is offset because you catch cancers earlier when they are less expensive to treat. A substantial cost savings will come through first line analysis of imaging by AI, to highlight images for radiologists to focus on.
But if the tests are cheap enough you can repeat them and compare. Bringing cost of blood diagnostics down is I think a good idea. I have fitness and health conscious friends who regularly get direct-to-consumer tests as a matter of course. In a few cases they have caught issues very early.
This is understanding there are some false positives, but being able to recognize them and learn to ignore them is better than not testing out of fear you overreact.
Repeating a test doesnt help. The blood result likely wont be different the second time. A false negative is a person for whome the test just doesnt work, no matter how many times you repeat it. (In reality, these sorts of important tests are run multiple times against the single sample before they print the results.)