Well put. We can actually detect well below 50 copies/mL in the laboratory setting as well; we don’t go below this to maintain adequate test specificity and sensitivity in the clinical setting.
Not to put you on the spot but from my cellular bio lab I took it felt like PCR should be capable of detecting literally a single example of the targeted bit of DNA in the sample. Is the problem with detecting below 50 that the sample is too easily contaminated or is it that the primers can spontaneously bind to things they aren't supposed to at a high enough rate to invalidate the results?