The only thing this fanbase should be concerned about is where Bargs fits. The team is just starting to gel, and then now amidst the trade rumours, we want / wish for things to be broken up. This is a great group of guys - let them gel for a bit.
“The saving of our world from pending doom will come, not through the complacent adjustment of the conforming majority, but through the creative maladjustment of a nonconforming minority.” - Martin Luther King
For example, Rondo can be better than Lebron when they're playing high stakes games. Is he always better player than James? Of course not. But he can raise his game up to that highest level, so I would call Rondo a cornerstone.
On the other hand, as much as I enjoy seeing DeMar's improvement this year, I cannot imagine that he will ever be able to outplay any of the best players for even a single game when it matters.
Other cornerstones right now: Durant, Paul, Westbrook, Kobe, Anthony, Nowitzki, Duncan (maybe still?), D Rose, Kyrie, Pau Gasol, Dwight Howard (I'll keep him here for the moment, but bad backs can be brutal), Noah, Randolph, Love, Rubio, Bynum (an injury prone cornerstone, the worst kind).
Potential cornerstones: Paul George, Steph Curry, Marc Gasol, Anthony Davis, Batum, Lillard, MKG, Wall, Cousins, Bledsoe, Monroe, and a bunch of other dudes with much slimmer chances (and I'd put Ross and Valanciunas in this category, with Ross ahead of Valanciunas).
And I agree with Mediumcore, I think you do whatever it takes to get any of these guys. You overpay for these guys.