I am 100% on the side of keep Jaylen if he wants to be here. Lock him up at whatever number he'd accept to stay with Tatum. There is nothing left for them to prove that they are a successful pairing, and one that can lead a team to a championship.
But lets not misrepresent the other side.
It's not a question of does a Tatum lead team play better without Jaylen on the court but rather would there be another player or players that could would work better with Tatum for Jaylen's trade value and salary slot. DeArron Fox, Trae Young, Karl Anthony Towns, Luka Doncic, Damian Lillard, Jaren Jackson Jr., Zion Williamson, maybe even Joel Embiid if Harden leaves and he forces his way out? The most interesting one in recent memory was Kevin Durant but now that's over. Are there players that could either be acquired for Jaylen in a trade that would be a better fit for a Jayson Tatum lead team.
Again, to me the answer is JKJB, but it's not like we'd be losing him for nothing if the team chose to break them up.