Firstly, I think it's pretty unlikely that this rumor will come to fruition.
Secondly, I wouldn't like it if it happened. I don't think Ryback has reached that level to be honest. Sure, he'll make a good main eventer for WWE for the next 10 years or so, but I don't think he should beat the Undertaker.
Another point; if they're going to end the streak at this year's Wrestlemania, then that surely means the end of Undertaker at Wrestlemania? There's not going to be any point in asking him back the following year, since he won't have his streak. Sure, they can ask him back for some other random match, but what's the point? And is that something that he'd want to do? I somehow don't think so personally.