I think Nash gets severely underrated by a lot of people because his shooting percentage has been low 2 of the last 3 years and because he adds value differently than a typical offensive star.
He's 21st in the NHL in shots on goal this year, behind only Burns, Ovechkin, Tarasenko, Seguin, Kane, Kane, Kucherov, Kessel, Eichel, Trocheck, Skinner, Pacioretty, Hall, Jones, Gallagher, Marchessault, Tavares, Bergeron, Josi, and Hoffman. That's a really good list of players. His goal scoring is lower because he's only scoring on 9% of his shots this year, which is the 2nd worst in his career and well below his 12% career average. But shooting percentage is a notoriously fickle stat that tends to mean revert, so it's very likely the low shooting percentage is just a fluke. At his typical shooting percentage, he'd be on pace for 31 goals this year, which I think is a more accurate reflection of what someone acquiring him should expect from him and makes him a very valuable player.
The 2nd thing to understand is that Nash gets a limited amount of power play time and scores almost all his goals at even strength. That would make him especially valuable to a team with a good power play like Pittsburgh, Tampa Bay, Winnipeg, San Jose, or Nashville, for example, because it means that more of his scoring will be legitimately additive or incremental, instead of coming at the expense of a player who is already productive on the power play (which is one reason why Shattenkirk wasn't a great fit in Washington).
In addition to that, he's great defensively, has playoff experience, and is a good locker room presence.
If I were Nashville, I wouldn't give up Tolvanen either, but I think concluding that he's not worth at least a late 1st round pick+ would be a mistake for a team that has a legitimate shot at the cup and could use another forward.