I disagree that dogfighting is relevant in modern warfare, at least with USA as one side.
That may be, but this is specifically a fighter plane. A plane designed to dogfight. That is the metric upon which it is being judged here.
Similarly, an ICBM is a poor tool to handle smugglers off the coast of the US. You can judge the concept of a fighter plane as irrelevant in 2015-2037 (the period over which they are being delivered), but your initial statement, "Why is dogfight a parameter in assessing 5th generation plane?" can be simplified down to, "Why is aerial combat a parameter in assessing a plane intended for the role of aerial combat?"
The answer to that simplified question is: Because that's the slot of functionality it is intended for.
But you probably meant to ask, "Why are we making 5th generation fighters anyway?" That's a good question, but I'd suspect that the answer is primarily because they are still used worldwide today in shows of force and occasional engagement. They are scrambled now when commercial jetliners go radio silent, ever since they were used as weapons on US soil.
Another aspect is that military forces are intended to be functional -- but also showy, so they can be used to intimidate. And intimidation is a tool of emotion, not logic. There are strange quasi-engagements between many countries on their borders to show intent to defend, and fighter planes are often used in that capacity. Being intimidating also helps your own forces. Fighter pilots are perceived as badasses, and a young person's gut instinct is to want to have the badasses on your side when you're being ordered to throw your body into armed conflict.
This is also related to why all branches still have swords as a ceremonial part of their formal uniforms, and they are used in situations like honor guards and events of historical or great personal importance. Military might is not a video game or board game with simple stats. It's sloppy and human, and involves more diplomats and mistakes affecting it than simple white room simulations tend to account for.