Morgan,
to answer that :
Ballas - no case to retain. Sure he tries and gives 100% but he goes missing for most of the game and constantly falls overs, a problem he hasn't addressed in a number of years.
Sandi - Yes, He is still the best tap ruckman in the game and serves as a great teacher for his up and coming replacements. Hodor can learn a lot from rucking alongside Sandi. Sandi as a ruck coach also has a lot of value.
Bennell is harder to decide. If he gets past constant injuries and off field problems, he might be a valuable asset. The only doubt is whether he will ever be on the field long long enough to contribute. Anybody who doesn't have large doubts about him hasn't been watching him. He must be on his last chance.
As to whether Cornes has made a point, clearly, in his own rambling way, he has asked questions that people should be asking. Titus Oreilly has stated things much more clearly. I don't know why you mock the messenger when anyone with half a clue would be questioning the conduct of the rebuild.