The downside being that Atletico would not sell they would only run the contract down since their would be minimal money for them in selling him. With a 5 year contract he would be 30 years old when the contract ends and his market value will be a lot less.
Besides the obvious flaws that you mentioned: Dortmund could probably offer him the same deal. And unless Atletico completely feck over their wage structure Reus would probably still lose €5-10m compared to what he could earn in England. So Atletico would probably have to pay Dortmund around €50m++++ to even make them consider selling Reus (imo at this point they won't sell no matter what the offer is, because they belive in finalizing their important business before the window opens) and then give him a release clause around the same number, so he can be sure to move on to a bigger club in the not so distant future and pay him like €8-10m a year and give him like 50% of his next transfer fee, because after all he'd move for the money in this case and some English club would probably offer him something above €10m a year and a nice signing bonus.