Forensics are not the only type of evidence.
There could be something written about it from the time, such as a diary entry or a letter to someone. That would still be a "he said, she said" situation, but it would prove that Ford did not make up the accusation now just because Kavanaugh was being nominated.
Other people could have heard rumors of the situation at the time. Once again, not "proof," but it again show that it wasn't something made up recently.
If he actually is a rapist, there could be other instances with other people, as with the President and his, ahem, "genital-grabbing." Some of them, if they exist, may now come forward.
And do you know for certain that the reason she does not want to testify before an investigation is because she does not want to perjure herself? What is the basis of this claim? Are there not other reasonable reasons for her not to want to go in front of a hostile inquisition board, with members who will lie and try to tear her story and reputation apart, without further proof to back her up? Can't you think of any other reason?

So it comes down to how credible people find her. And her credibility won't be diminished if she isn't heard and the charges aren't investigated.
That's why this is pure politics. You just articulated a standard that a believable liar could meet. This should come down to proof, and there isn't any.
Except that you already stated that there is no longer proof. So if Kavanaugh actually tried to rape her, are you suggesting that we should ignore it, because there no longer is proof?
That's a standard any rapist would love.

Now the ball is in the Republican's court. You gotta tell us a solid, legitimate reason not to delay the confirmation a little while and at the risk of putting a rapist on the Supreme Court. 
You understand that she alleged she was groped. Interesting how you jumped to rapist from that.
I understand he tried to pull off her clothes and panties. Since she happened to be wearing a one-piece bathing suit under her clothes (having come to the party after swimming), he was unsuccessful. That sounds like attempted rape to me, not just groping. If the story is true, that would mean he was a frustrated rapist.
Now I'm not saying that he did try to rape Ms. Ford. Right now, we just have these two stories. Frankly, Kavanaugh does not look like the type of person to do such a thing to me. But these are serious charges and should be investigated thoroughly, even if there probably is no evidence any more, just in case there is somewhere. Rushing the confirmation through doesn't help anyone.
Except for Republicans.
So tell me again why we should rush the confirmation through, and possibly put a rapist on the Supreme Court, when it just benefits Republicans?