If you trade him, what are the odds you get a player of his caliber back that wants to stay in Portland. Ben Simmons doesn't want to be in Portland. He has made it clear, at least in the past, that he wants California. So you trade for him, and he leaves as a FA or (shocker) demands a trade. You think he will all of a sudden act like a different guy once he lands in depressing Portland? He would fucking hate life as a blazer.
That means you gamble on young talent/picks. Again, what is the likelihood they a) will be all star level in a few years (I say 1 in 10), and b) most importantly, want to stay in portland (I say 1 in 10 again).
Even if we are generous and say 25% chance for each scenario, that's still just a bit better than a 10% chance you do as well as Dame if you trade him. 10% chance is generous IMHO. You won't get the talent, loyalty, and character in a single guy. Odds become very very small.
You are better off keeping him and using his loyalty to attract players that want to be in Portland.
Honor his loyalty, don't fuck him over. He says time and again he wants to be here. Keep him. The odds are, you won't do better than Dame if you deal him.