Evidence of the NSAKEY being a backdoor includes some description of how the backdoor might work, backed up by a reference to the relevant Windows source code or its disassembly, both of which are easily available to researchers. What sort of backdoor is it? Does it provide remote access to Windows? Does it enable certain cryptographic modes that are disabled? Does it disable certain cryptograph modes that are enabled? Does it trigger key recovery, and if so, how?

Evidence of X does not include "X would have been done by Y, and Y did Z, and X and Z are both bad, so why wouldn't Y do X too." That is basically the definition of an ad hominem argument. Whatever else the NSA may have done, and however much it's reason to believe the NSA might have wanted to do this specific thing, it's not evidence of them doing this specific thing (and again I'm not sure what this specific thing is even supposed to be). And if anything, the lack of mention of NSAKEY in the leaks is a reason to believe that there wasn't anything there.

Evidence of X also does not include "Y refused to talk about X." That might be evidence that Y is suspicious and untrustworthy (or evidence that the person asking was a conspiracy theorist who wouldn't be satisfied by any explanation), but it's not evidence that Y actually did X.

So, that's my definition of evidence. I'll turn this around: what would evidence that NSAKEY was not a backdoor look like to you? Would anything convince you, or is your claim unfalsifiable?