My uncle bought a ninja 900 like the one Tom cruise rode in top gun(late 80’s) and when they were testing that engine they ran it at redline for like 24 hrs. Bikes these days are designed to run everywhere inside the redline for extended periods. I suspect that the redline has to do with where the power dropoff occurs because as was mentioned before, what’s the point to continue running the revs up if you’re losing power. I’m sure this engine could run at higher rpm but the 6k redline prob keeps it bullet proof and closer to the power band.