Seems kind of ridiculous to analyze that far. Getting too far into the "what-ifs" now.
They had $7.6m tied up in Burakovsky, Hagelin, Connolly and Jaskin. I think they could have easily fit Forsberg in, and probably convinced him to take a bit of a discount, or at least sign him on a two-year contract instead of a six, as to not impact depth.
Furthermore, they could have even explored trading him after he broke out, and gotten a significant haul.
Forsberg "leading" Nashville to the cup has nothing to do with this, as no singular player leads a team to a cup. Not even McDavid can, though, Forsberg had proven to be a quality producer in the playoffs the two years prior to WSH winning the cup. Maybe they win it even earlier with him in the lineup?
I get it that the Caps are happy at the end of the day because they won the cup. That is very fair. However, they still traded away a franchise player that's probably going to hit 1200 games played and 1000 career points, for less than 30 pts. This we know concretely as fact.