you are the rightful owner (unless you stole the wallet)
Why?
Why do you choose to draw the line in the sand there? If you follow your line of argument to it's logical conclusion, every reason you could give for a software glitch transferring legitimate ownership of the Bitcoins could be applied to someone else coming into possession of your wallet.dat as well.
You just choose to draw the line at wallet.dat thievery, while others draw it a bit further back.