How is Julius Caesar significant when he dies so early on in the drama?

Many readers have questioned why Shakespeare's play should have been called Julius Caesar when Brutus seems like a far more important character as well as a more important role for an actor. It is true that the actor playing Julius Caesar is offstage for much of the time. Perhaps Shakespeare reasoned that the real Julius Caesar was such a dynamic figure, such a colossus, that his presence could be felt regardless of whether he was onstage or off, and even regardless of whether he was dead or alive. The first two acts, leading up to the assassination, are all about Julius Caesar. He may not be present, but everybody is talking about him and thinking about him. Even the gods seem greatly concerned about him. Then when he is lying dead, Mark Antony addresses a famous soliloquy to his fallen friend in which he prophesies that the outcome of this assassination will be years of civil wars.
Over thy wounds now do I prophesy
Which like dumb mouths do ope their ruby lips
To beg the voice and utterance of my tongue,
A curse shall light upon the limbs of men;
Domestic fury and fierce civil strife
Shall cumber all the parts of Italy;
Blood and destruction shall be so in use,
And dreadful objects so familiar,
That mothers shall but smile when they behold
Their infants quarter'd with the hands of war;
All pity choked with custom of fell deeds,
And Caesar's spirit ranging for revenge,
With Ate by his side come hot from hell,
Shall in these confines with a monarch's voice
Cry “Havoc!” and let slip the dogs of war,
That this foul deed shall smell above the earth
With carrion men, groaning for burial (Act III, Scene 1, lines 279-295).
So the play continues to be all about Caesar. His assassination motivates Antony to start a mutiny which drives Brutus and Cassius out of Rome. It brings Octavius to Rome and eventually leads to the battle at Philippi in which both Brutus and Cassius lose their lives. The ghost of Caesar appears to Brutus twice to inform him that he will be present at the battle and will be responsible for Antony and Octavius's victory.
Caesar is present throughout the play. The play is about how people are afraid of him, how they conspire to kill him, how they actually do kill him, how Antony uses Caesar's body to turn the mob against the conspirators, how Antony and Octavius take power as Caesar's obvious successors, how Antony and Octavius pursue Brutus and Cassius to take revenge against them and the other conspirators for the assassination of Caesar, and how the new Roman government is shaped by the government Caesar himself planned to form. Julius Caesar dominates the play from start to finish because of his awesome willpower, a power he exhibited all his life, as can be seen in his many conquests and relentless hounding of Pompey to his death in Egypt. Brutus is a noble and courageous man, but he does not have the superhuman charisma of Julius Caesar. In this regard, none of the other men, friends or foes, could measure up to Caesar.