I think if the Rangers aren't giving back a solid goal scoring forward, the Caps probably settle for Alex Edler, who also plays 23 minutes, also has about 20 points, also starts most of his shifts in the defensive zone, and probably is a heck of a lot cheaper:
https://www.hockey-reference.com/players/e/edleral01.html For three first round picks to the Rangers, the Caps should be brining in McDonagh, Grabner, and Zibanejad. Because that's how rebuilding works for most teams in hockey. You give up a solid roster player to get another team's first pick and maybe they throw in a prospect to help tide them through until that pick pans out. But when teams decide to rebuild, it's not so much about the quantity of draft picks, as it is about breaking the team down to where it can get one of the best players available, for a couple years in a row.
If the Caps can't get even Edler for Johansen, then I think they keep Johansen and let Djoos play in the playoffs, with Chorney to back him up if Trotz decides to bench him for a bit. Even with retained salary, they're probably not picking up a Duncan Keith, Marc Staal, Dion Phaneuf, or Karl Alzner, although the latter would be pretty funny, and a couple of Canadiens fans have suggested it. Actually, yeah. Let's go with that.
If McDonagh were signed for seven years, at 5M, sure, three first rounders. But for a year and a half, two is already a ridiculous overpayment.