If the Germans had kept the Kaiser, there would be no Hitler.
When the Nazis took power, they absolutely forbade the exiled Kaiser to return to Germany & he died in exile in a house provided by his friend, Queen Wilhelmina of Holland.
Kaiser Bill respected Dutch neutrality in WWI. When he died, the occupying Germans placed a guard of honor around the Doorn house. Hitler ordered it immediately withdrawn.
Crown Prince Ruprecht remained a popular choice for monarch well after WWII. Where are his descendants?
Exactly. And possibly no Lenin.
We fought against the Second Reich in WWI, led by the last Kaiser, who abdicated. No reason to conclude that the absence of a Kaiser was a cause for the war that followed.