Have we though?
So far we've had Lloris, Rose, Bentaleb, Mason, Dembele, Chadli, Eriksen, Pritchard and Son injured at the very least.
It could have been worse, but I'd say that 8 of our top 15 players missing games due to injuries in the first three months of the season isn't anywhere near lucky.
Yes, backup is good, but if anything we've proven that we do have good backup. We don't know yet if we have good enough backup up front, but personally I believe that we have the versatility to be ok should Kane be injured. Of course, we don't have the backup to be as good without Kane as with him, in his current form, but I don't think we'd be better off with West Broms backup as our backup.
And, and this is very important, especially for a club that wants to develop young players: Too much backup is a thing, and can easily be a problem.