It's a medical discussion (sort of) so I can't bring up Occam's razor. However, I can explain a standard titrated plaque assay as a self controlling experiment of CPE, with a higher dilution of the virus solution producing less plaques. You continue to search using efficient methods (something close to a modified binary search) for the titration that gives a reliable but still descrete (so not overlapping or filling the whole plate) set of plaques, and then write down the dilution that gave you that result. Viruses in the original solution cause the plaques. The viruses can be cloned, giving high confidence you have the right one. As you said before, you don't trust computer methods, but using modern computer squencing methods you can actually check the viruses on the plate, to make sure the sequence hasn't changed too much. I want an explanation for this process that does not require viruses being real. Remember, cloning prevents things like prions and bacteria from contaminating the original solution, and the titrations control for experiment caused CPE, as you'd get no signal if that was all there was.