For those who don't want to click through, the question is

All of these, now, can be explained very simply without any Q document. Here's how: simply assume that Matthew borrows text from Mark, and that Luke borrows text from both Mark and Matthew. This seems like a much simpler explanation to me, as it doesn't introduce some mysterious document that was popular enough to have been in the hands of the authors of both Matthew and Luke, but which has never been cited by anyone. What's wrong with this logic?

Here is a diagram of the two-source hypothesis.

Notice that your hypothesis does not work because some of Q which exists in Luke does not exist in Matthew - Luke pulls from Q in places where Matthew does not. This is because the Q document is not reconstructed from the canonical gospels alone, but also from many texts which we have from the time, many of which also make reference to a shared body of sayings, which we then call Q.

As for the attestation, not having Q, if it was ever even a written document, is not particularly surprising. We are missing a great number of texts which we have attested in other sources from this and many other periods.