If you wanted to re-sign Harper, it made zero sense to trade him. One, it might piss him off, two, it gives some other team the chance to woo and recruit him for two months, which is the main advantage the Nats have. It is clear to me that the Lerners want to re-sign Harper, which is why, because no one gave up the keys to the kingdom, he wasn't traded.
It makes even less sense to trade Rendon if you think you will be competitive next year.