If Harper was coming back it wouldn't take that long to decide.
If there is mutual interest on both sides for Harper returning to the Nats, and I think there is evidence to suggest there is at least some on both sides, then it would likely play out in one of 3 ways.
1. Nats give their final offer, and say take it by the end of the world series or it's off the table.
2. Nats give their final offer and let Boras shop him around, but say the offer is always good.
3. Boras shops Harper around, gets the best offer and comes back to the Nats with the ability to match or counter his best offers.
Its seems that something like 3. is probably the most likely scenario and probably leaves the chances of the Nats signing him around 20-30%. The Nats do have a little bit of an advantage in that they don't have to give up draft pick compensation to sign him.
It seems very unlikely they would have held out this far and not at least see what offers are out there. There is no benefit to Harper signing now.