Originally Posted by Barry2

Going back to your original question.
That broken shaft is integral to the spider and crank arm on the other side of the bike. (spider holds the shainrings)
The shaft itself cannot be replaced.
At minimum you would need to replace the other side... Shaft, Spider, crank are all one piece.
It might be cheaper to replace the whole Crankset (left and right sides) if you can find a deal.

The bottom bracket bearings in the frame, were likely damaged by the shaft failure. So it's a great time to replace those also.
Not replacing these bearings "may" damage the new shaft if the old bearings freeze up.

Hope that helps
All the best
Barry

EDIT: BTW I recently replaced my 10speed FC-6700 crankset with FC-R8000 with no issues.
Shimano has retained compatibility.for the cranksets