I think we need to see how the season plays out before we try to make any ultimate determination about Frost. Things are definitely trending negatively, but I would like to address a few of the points made by the OP.
1) The fact is, this team is light years more competitive than it was 2 years ago. Yes and no. You can only make this argument if you want to look at very specific parts of the team, in which case this becomes a weak argument. And the W/L record suggests they're right where they've been all along.
2) But, he has shown over and over again that he will fix that. I don't see enough evidence to support this claim. Special teams overall has been a sore spot for four years. The offensive line has been arguably the weakest position group for four years. The running back room has been fairly inconsistent. Unacceptable penalties. The defense has certainly improved, but we also know that Chin is more responsible for that unit than Frost. I'm struggling to see evidence to support the notion that he has fixed these types of problems "over and over," because the more fair argument at the moment is that he hasn't shown that... because they continue to be problems.
3) And that is true, but St is looking to be a better team than we thought they were a month ago. While true, the tougher pill to swallow is that - once again - Nebraska looked the better team, particularly with the overall dominant defensive performance. Teams perform better and worse than their pre-season projections all the time. That's old hat. What matters is how you play against them.
4) I suppose if we meltdown and lose our way to the end, there may be no choice but to make a change, but we owe it to the program to let it play out. Ultimately, I agree with this. I've become fairly skeptical this year to the point that I don't think they're going to get much better than where they are. I think a best case scenario is bowl eligibility. Even winning four games will probably buy Frost another year. Finishing the season with only two or three wins will probably be a bit of a death sentence, even if the team is competitive in those games.