My opinion is: regardless of the team's outlook, no competent GM should trade their best players out of sheer value. So for that, I don't fault Blake.
The whole purpose of the pipeline is to develop players to supplant the vets. You keep the players who fulfill their roles well and challenge the youth to replace them.
If the players you draft and develop can't outplay aging vets, you either need to evaluate your drafting or your development.
Byfield so far has shown promise, as has JAD up front. Many of the young blueliners like Anderson, Bjornfot, Roy, etc have established themselves as competent mainstays. There are still issues with the blueline, and they should be more experienced and prepared next year.
Up front is pretty messy. I expect JAD, Byfield, and Vilardi will be more consistent and cement themselves. On a phone, but won't get deep into why they get passes from me. Grundstrom, Andersson need to establish their role/identity. Everyone else up front... they are what they are, for better or worse. That's not to say they're bad, but their roles and expectations are fairly easy to establish. They either play their role well/more consistently, or we place the expectations of some prospects (Kaliyev, Fagemo, Thomas, etc) to replace them.
I don't blame Blake for holding onto the core, provided he felt they fit his vision. But he has had a few years to see his vision take hold. If the Kings can't make the playoffs, then he either had a poor vision or he executed it poorly.