Even if Archduke wasn't killed a cause would be found in some of the colonies, such as Morocco.
Explanation:
WW1 was to happen no matter what.
Germany and Austria-Hungary wanted to gain control over certain areas, and especially wanted more colonies.
This was seen during certain crisis - Lebanon, Morocco, etc where they have shown this.
The only question was when the war will start, not will it start. Even if this didn't happen, some problem in colonies would erupt, and at the end it would lead to war.
