Why would it limit them with Bryant? There is no rule that says a team can't have $60-$65M/year tied up between 2 All-Stars.
But Harper is likely getting 40 Million a year and Bryant even with four years of arbitration likely will break the arb record his first time through. If he stays healthy it will get spendy really quick, and toss into Heyward and his immovable 21 a year, plus Lester for at least two years at roughly 23 million, and it's not 65 million for two players, it's 100+ million for four. Tack on those other guys about to hit arb as well, it's like the Nats in 2012 forward. Those good pieces that are cheap won't stay cheap for long, so unless you have a farm system that will keep pushing out prospects that are very good to elite, it's hard to keep that money wrapped up in a few players. The Angels, Giants, and Old Yankees are perfect example of this.
The Nats have a good roster moving forward, with a cheap OF in Eaton, a young elite prospect CF in Robles, a steady, and still pre-arb SS in Turner, an anchor at 1B, and hopefully a step in 2B in Difo (also cheap) and some trade chips to move for controllable pitching or a 3B replacement when Rendon leaves. Heck, maybe Soto brings back a 3B or Kieboom turns into that guy. I think the Nats have a more compelling argument to keep Harper and given the way the ownership likes to spend, they would be more likely to open up to thier big money draw, Boras client.