How long is a piece of string eh...
It could be a small-ish knock by your standards, anywhere, maybe a slight crack, but the impact might have been enough to damage it.
The cracks in the back, whilst mostly not that bad, it's the ones on the front, and you are through the screen onto the digitiser part of it, then thats mostly a goner.
Screens and digitisers are usually just as one now, and anything major happens to the screen is bad.
I don't know how much from Sony, I dread to think what an our of warranty repair would cost(would they class it as out of warranty, as it's not their fault?), but to buy an OEM Xperia 1 screen on the few places I looked was around £200, and I doubt that was legitmate, TBH.
Best bet is to get your phone insured either through your house insurance, or a policy, if you didn't take the ones networks usually push.
My familys electronics are part of an extra on the insurance, but worth it, as we've had a couple of claims over the years, with heavy hands!! 😃
