AJ vs Shane is definitely a strange choice for both guys.
AJ is a heel who is over as a face and Shane is a face GM who is less over than he was upon return now that the shine is off. Shane hasn't really had any storyline involvement in months either.
So the crowd is gonna be split but probably in favour of AJ.
AJ has maybe 2 or 3 more WMs in him, also. His first one was nothing, and this one might end up being nothing, too. He will probably never be in the WM main event but he should be having a 30 minute match in the semi against somebody almost as good as he is.
Shane could have been put against Miz. Miz gets actual heat and people would love to see him embarrassed by Shane. The whole thing with Miz is he's a bad wrestler despite his ego, that could've worked. Imagine Miz cheating to get the better of Shane, leading to a DB run in and interference. Could've worked pretty well.
Another strange point about it is - AJ is known to the hardcore fans, who will not want to see him wrestle Shane very much, and Shane is known to the lapsed fans and casual fans, who will either not know who AJ is or not particularly care. They kind of cancel out each other's audience.
I think Vince and the others overthink this stuff. Why isn't Cena fighting Undertaker, either? Why did Charlotte lose her PPV streak a few weeks before WM? There's a lot of counter-intuitive stuff they do and I couldn't guess why.