If I were you, I'd be worried (assuming you are a Nats fan and not some crazy Phillies fan lurking around. If you are a Phillies fan and I were you, I'd be ecstatic and possibly covered in steak and cheese.)
This is more than simply a deal for a premier pitcher for a team looking to win a title. The Phillies are not a desperate team. They just went to back to back World Series. Their core players will all be 31 or younger next season. They had Cliff Lee on their roster for one more year, numerous other decent FA pitchers this year and particularly next year they could go after, and some highly sought after young pitchers already in their system. This was a team that already was going to win. This was a team that already was going to challenge for the pennant.
This is a move to keep the cycle going, the cycle of winning. Winning leads to more revenue which leads to more spending which leads to more winning. This is a lesson a team like Boston has learned very well. Boston is only 10th in population yet it is consistently has spent in the Top 5 of teams in terms of payroll. Consistent winning has brought consistent interest, which allowed Boston to generate consistent revenue. Boston's revelation (along with the Yankees being the Yankees) has turned the AL East into a 2 team show for the past decade.
This could easily happen to the NL East as well. The Mets can't spend like the Yankees, but they could (and have been) spend like a team in that next tier down. If Philly continues to win and creates a Boston situation, the Nats could be facing the same challenge every year that their "hated rival" Orioles face. An near impossible one.
A dominant team can and will slip up every couple of years. Injuries will happen, along with surprise player collapses, players not developing on time, even bad luck. Occasionally teams will underperform. A well-developed team with a moderate payroll can slip in during those opportunities. But the chances of two teams, both committed financially to winning, both run with a modicum of smarts, failing in the same year? That window opens like once a decade. Yes there is always the Wild Card, which should be there with more frequency, but again a team in a division like this is starting behind the 8-ball. They play more games against these power teams then teams in other divisions do. Their wins are harder.
There is hope, of course. The minute you make a couple bad decisions and/or your owner won't throw money at the problem the cycle can collapse. It takes doubly long to build up than to tear down. Look at Cleveland. Dominant in the Central from 1994 through 2001. All it took was two bad years and the cycle collapsed. Attendance plummeted, revenues crashed. You have to believe in this cycle fully. There can be no multi-year breaks to refresh the roster. You can't have bad years back to back. You need to be competitive nearly every season.
The Phillies are on the edge of something special. The base is set. If the Phillies can turn these next few years into something more; a title or two, a network, a national fan base and then commit to keeping it going, the Nats could find themselves staring in the face of a mountain to climb year after year.