I don't think Tom was weak. Not everyone in the family needs to be a brute or a tough guy. Apparently, like anyone else in his position, he organized or passed on orders for murder and mayhem.
Tom had no problem having violent disputes with Sonny and didn't mind occasionally disagreeing with Vito. Those two people loved him as a brother and son. Michael did not.
It probably became clear to Tom even before Fredo's demise that it was not out of the realm of possibility for Michael to kill him. (In book he says this explicitly ) So of course the relationship between Tom and Michael will be different.
Tom's removal as official consigliere was ultimately Vito's idea though after Vito's death Michael kept that decision I think more as a way to punish and control Tom than any sort of honest business decision.
"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming
Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.
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