It obviously depends on the trade, for me. I certainly wouldn't do it for A-ball lottery tickets. However, while we aren't rebuilding I think it's fair to say that we could be reloading. We have 4 more years of Strasburg/Scherzer/Eaton/Trea/Robles and if re-sign Rendon I'm basically fine with that being the heart of the team. I don't believe that equals 2018 or bust, especially in this terrible division at this point when even 89-90 wins could be a division champ over the next 3-5 years.
Let's put it this way - last year the Brewers won 86 games and missed the playoffs by a game, while the Rockies won 87 games and made the playoffs by a game. The Brewers offense + pitching was worth 34.6 WAR and the Rockies offense + pitching was worth 31.9 WAR.
So if you assume that those 6 guys above should average 4 WAR apiece, that's 24 WAR, which is like 2-3 more WAR than the entire Orioles team had last year when they finished with 75 wins. So it's not an impossible situation or anything to add 12-15 more wins from the other 19 players on the roster over the next few years, although it's a lot less likely we're going to have 98-win seasons and a lot more likely we'll have 88-90 win seasons instead.
Now could those guys crater and not average 4 wins apiece? Of course. It's also possible Juan Soto ends up being the best player out of all of them. My point is simply that Bryce Harper isn't the end of the talent on this team, and if the right trade comes along - say a near-MLB ready 1B with a lot of pop 
- I don't think you turn it down just because 2018 is the last year we can possibly win. It's definitely not.
how much longer can you count on a terrible division? Likely another year, but eventually prospects mature and both the Phillies and Braves have strong farms
