I seem to recall the Moslems owned Sardinia, Corsica, Sicily, Spain, Crete, Greece, Albania, and a swath of the Balkans, and a huge chunk of Hungary, and then they got gob-smacked back to the heartland in the Middle East and North Africa.
That happened long before WWI.
You make an excellent point. Too many people see current trends, and assume they will continue uninterupted. This is frequently wrong. We will have to wait and see.