If they can use the DNA from picogrammes of DNA, ~15 cells, worth I suspect it would be hard to prevent leaking DNA during a crime; a bit of spittle, a cough droplet a tiny flake of dandruff could be enough I'd guess, if not now then soon.

DNA just says the person was present. Something that small could even come through an open window from miles away. You generally need the DNA source to be damning, like semen, skin under fingernails, etc. For it just to “be on the couch” might get you an interview with the police, but not a conviction by itself.

Also if you talk about picogrammes those might have been carried to the crime scene by somebody else.

Imagine you're condemned to prison because you sat on the same train as a soon to be murder-victim, and your dandruff landed on their clothing.

While this may seem technically promising, it's also likely that the growing volume of cases will include enough false convictions to cast doubt on the enterprise. Especially if more shady businesses get into the game.