I admit, I am 50/50 on if we have seen the final match of The Undertaker or not. And with no formal announcement from WWE or Mark Calaway himself, the door is still open. I honestly feel there is still a good chance Sting vs. Undertaker at Wrestlemania 31 may still happen. And here are a few reasons why....
1) As I said, no formal retirement announcement as of yet
3) I think Undertaker would want to go out on a higher note, even if he loses his last match. It is no secret Undertaker was hurt during the match with Lesnar. As a result, this was not one of the classic Undertaker matches as we have gotten used to at Wrestlemania. Wrestlemania 30 being Undertaker's last match is like he went out with a little fizzle and not in a blaze of glory.
I await other's opinions on the subject.