Note: I am not advocating anyone continues using SHA1-signed certificates: they are dead as far as security is concerned and should no longer be used. I'm just trying to clarify my understanding of the theoretical implications of Shattered as they might apply to such certificates.
Prompted by this recent question: After Google's collision attack, is RSA-SHA1 signature still safe?, to which the overwhelming response is "no", I want to ask to what degree does the Shattered result actually show that SHA1-signed certificates are "in danger"?
The existence of the Shattered result is not, I think, in itself a surprise: everyone knows that in theory you can create two streams of bytes that hash to the same value. Google's achievements (which I don't wish to downplay) are (a) that they mustered enough resources to actually do this, and (b) they did so while keeping the colliding file a valid PDF (that also matched the format of the original very closely, and included the "obvious" difference in background colour).
As the accepted answer to Does “SHAttered” only work for PDF files? explains, Google had the "luxury" of an effectively "free-form" block of data that they could alter at will to achieve the collision. As I understand it, the "collision blocks" embedded in the JPG data (itself embedded in the PDF) have no (visible) effect on either the image itself nor on the PDF. Therefore (to grossly simplify things), "all" they had to do was vary this block until a collision was achieved1.
However, in the case of a SHA1-signed certificate, there is nothing like the same freedom to inject unconstrained data: the amount of data that is signed is much smaller; and far less of it is free to be varied without consequence – you might find a collision, but the result is likely not to be usable as a certificate.
So: is it correct to say that while Shattered was a significant event, it does not in itself show that the chances of creating a usable, collided, SHA1-signed certificate are increased, and, in fact, the chances of doing so are still small?
1 As per parts of Maarten Bodewes's answer, I realise that Google's attack wasn't completely random: the "magic" was in their tailoring of the attack and an "academic level knowledge to construct such an attack". While the targeting was important to being able to find a collision, so too was the ability to put arbitrary data into the file without compromising the file's validity. It is the latter aspect I'm asking about, and whether the same freedom extends to X.509 certificates.