Eaton is two years younger as well, and the Nats must be somewhere in the vicinity of the luxury tax threshold, right? So, there might be a cliff effect of staying under with Eaton but going over with Fowler.
And you're giving up a first round draft pick too. So, that's equivalent to at least 1/2 of one of the players given up.
Nats have $116.5M committed next year with about ~$29 M projected for arbitration eligible guys after non tendering Revere. (Note this is salary, not aav for some reason I'm having trouble finding aav's for next year on the google box). Werth comes off the books after this season. Murphy after next. Zimm the following year. So the argument can't even really be made that they're saving money for a Harper or Rendon resigning. They have plenty of room.
In no world is the 28th pick in the draft worth half of Lucas Giolito or Reynoldo Lopez. If for nothing else than proximity to majors, development invested and probability of being a contributing major leaguer, I find it difficult to believe an argument can be made to that effect. Someone may like to construct an argument about draft pool money, etc, but I think it's likely a stretch at best. I'd gladly entertain it though.
Eaton is a good player. His biggest attribute is his cost control. But he's miscast in CF and that's what the Nats needed. He's far better suited to RF, so maybe they're planning to make Harper the CF, which would make the trade look better. But either way, if the rumors were true and this package was what was on the table for Sale - Robles included or not - and they sent it for Eaton, than Rizzo most likely overpaid by a good margin and would have been better off giving that contract to Fowler or offering that package for another CFer. As a Sox fan, I'd have gladly given JBJ for either, let alone both. He has had a remarkably solid track record, but this seems like it might have been his first big miss. The contract for Fowler will almost undoubtedly be an overpay, but the price paid for Eaton will almost certainly be more painful in the long run, even if you don't buy into Giolito being as good as he is touted.