Even if they eventually send any, what difference is it going to make? Russia is planning a massive offensive. Those jets won't be there when ukraine needs them. Plus it takes a hell of a lot to deploy air platforms. I don't know how well the S-400 Russia deployed in Syria worked. The Israelis seemed to have avoided them in their bombing runs across Syria. So I'd view the S-400 and S-500 to be legitimate reasons why the West has been hesitant up to this point. The F-16 and Rafael haven't really been tested against near peer anti-air opposition. While this may look like a great time to test them, I'm not sure anyone will be pleased to see their vulnerabilities exposed by the Russians even if the US has more advanced platforms.