I don't think Hagen was quite leadership material (no knock on Hagen by definition most people aren't) and just as importantly I don't think even if Hagen had been well suited for the job that he would have been accepted by the other Family members, not to mention the other Dons.

"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.