Unfortunately, a lot of the arguments the OP made sound like the Tremaine Edmunds decision all over again.
I don't like the thought of giving out 2nd contracts based on OK-to-good performance plus "growth potential"
He's had 4 seasons to show what he's got, and he hasn't yet been "great", which is really what you want and need if you're going to spend the big $$ on DE
I see very little down side to allowing Rousseau to effectively bet on himself during his upcoming 5th year
If he's amazing, you're happy to pay him top dollar
If he's just more of what we've seen so far, you swallow your pride, let him go in free agency, recoup a comp pick, and you have more bites at the apple