Risk.
Why risk watching or waiting for a decline.
If you trade him for a prospect package, you free up the close to 20m you'll likely have to pay a DH. You spend that money on an area of need and use the prospects to provide further flexibility to the farm.
I know prospects carry significant risk, moreso than Edwin all of a sudden declining. But you would still have the 20m you would've spent on Edwin, to spend elsewhere.
And honestly, I'd rather have Jason Heyward at 25m per on a FA deal and have a couple of really good prospects plopped into the farm as a return for Edwin, than I would watching Edwin DH into is late 30's and possibly decline.