I'm having a hard time interpreting this; are you saying that because people who believe in religions don't unanimously agree that implies all relgions are false?
In terms of core beliefs, I think your estimate is rather low. Obviously it depends on how you weight it, but the core beliefs of the Abrahamic relgions and even Hinudism and Buddhism have a lot in common.
> At the very least N-1 of N religions are wrong
I will take it further than this: 100% of all beliefs are at least partially wrong. Ultimately, one has to ascribe weight to something.
The problem is not the things they have in common, the problem is the claims for which you can't figure out the truth value in any way, regardless of whether they're shared or not.
> I will take it further than this: 100% of all beliefs are at least partially wrong.
Fortunately most beliefs held by people are not considered dogmas. Even Newtonian gravity was replaced when time arose for that. You didn't get a schism in form of a Newtonian Church splitting off.