I don't really see it as zero sum. AMZN has committed to 25,000 jobs there (there are only at 1,000 now) and has demonstrated that commitment very recently by voluntarily expanding the campus by paying up for some very expensive dirt (the hotel). Virginia Tech's campus was just approved. I see no evidence of AMZN pulling back from their commitment.
I think it's hard to argue that having 25,000 people making an average of $150K, working (at least part of the week) next door to your asset base / development pipeline won't be a good thing. there's also lots of public $ going into the area for infrastructure, metro, stuff, making it less damn ugly, etc.
National Landing/DC area will be a metro-connected dense millennial gen z or whatever playground where you can code to sell cloud services, lobby, lawyer it up, kill people with drones, save the world working at a non-profit, etc. and JBGS will probably benefit from that...eventually.