Agree. Murray's expectations were ridiculous. None of those guys were ever going to elevate their games enough to be considered core pieces no matter how much Murray complained about it to the media. And it's not their fault. Like you said, they're much closer to complimentary pieces than core pieces.
Rakell's game fell off massively and at times it seemed to be effort/dedication. When he was scoring 30 goals, he sure seemed like a core guy. But then he fell off a cliff. Too much of a passenger.
For Lindholm, I think absent injury he is absolutely a core piece. A top shutdown guy even today. A great guy by all accounts, but too quiet to be a locker room leader it seems.
Manson is not a core piece unless you consider "intangibles" - and even then I don't think he is. Then add the injuries and drop off in his game. He's got value on a good team that's looking to compete for a cup - not so much on a rebuilding team.