My point about the population of 500 foreign objects is not about the origins of the objects. Rather I question how the objects are being examined.
If you want to know something about a very large population it might not be possible to survey each and every member of that population. Instead you examine a sample of the population that you hope is representative of the population. You do this at the risk of sampling errors which result in findings that do not accurately represent the population.
Given a population of only about 500 objects, why examine carefully only 5% of them? Why take a chance that the sample does not represent the population? Why examine in a way that undermines confidence in your findings? Do they lack the resources to do the job?
The investigators say they have narrowed the possibilities to two scenarios, a ground to air attack and an air to air attack. A population of 500 foreign objects might be large enough to confirm one of the scenarios. But their approach can lead to undesirable outcomes, namely that they reach the wrong conclusion or (as seems to be the case) they reach no conclusion. They've asked the USA and Russia for more evidence. The USA and Russia might wonder why give the investigators more evidence when they don't utilize fully the evidence they already have.
A population of 500 objects might reveal not only the method of attack but also the sequence of events as MH17 fell to earth. By limiting themselves to a sample of 25 objects they are limiting what they can learn. Why?