Could they have successfully run tests on smaller sample sizes? Possibly, but the target market size is small (needing less blood isn't particularly advantageous unless you're dealing with children or babies without much blood in the first place).

The finger-prick-testing was always very likely to fail; the biology just doesn't work out in your favor. However, getting finger-pricks working was the entire basis of their strategy, not the "test using less blood" idea.