I totally get what you're saying and really wish I could agree, but I don't. I think there is a huge difference between having competitive teams that are entertaining and being truly successful. Of course, it all depends on how you define success. In my mind, the goal of any NHL team or any rebuild, is to ultimately win championships. So, (from your example) if Nashville and San Jose don't end up winning a championship with the cores they have, then yes I would consider it a failure (at least in the macro, big picture, overall sense).
Teams and rebuilds can have successes along the way. Goals for one season compared to another can change. Barometers for success are relative when broken down into steps of progression. For example, this season the DRWs goal was not to win the championship. The goal was to see a significant development from young, core players that we hope will eventually form the nucleus of a championship season. So, in that sense this season can be viewed by some as a success (despite their record indicating failure). Next season, the barometer of success changes. To some, next season will be a success if they make the playoffs. The next season, winning a round etc.
However, all of these small successes are steps along the way to the penultimate goal of winning a championship. If this rebuild goes all the way to the DRWs being year in and year out contenders and they reach the finals only to lose it'll be a fun and entertaining ride with a series of successes along the way. But if they never achieve the overarching goal of winning a Stanley Cup, then to me it's a failure (by the narrowest of margins, but a failure none the less).
The goal is to win the cup. It take a ton of little victories and successes along the way, but the goal is to win. I'm not saying failing to win a cup (in the above scenario) is the same type of failure and being perennial chumps like Edmonton. There are varying degrees of failure. But getting a silver medal, albeit better than all but one person/team in your heat, is still a failure because the overall goal was not achieved.
To me, it's all relative though. What's a success to one isn't necessarily one to another