There was never any real possibility that a victorious Prussia/Germany would not take *some* of Alsace-Lorraine. The question was how much.
"The annexation of Strasbourg and of Alsace appeared to him [Bismarck] necessary in order to enhance the security of southern Germany and make any French pressure on the South German states impossible." Hajo Holborn, *A History of Modern Germany: 1840-1945,* p. 222. https://books.google.com/books?id=Y4pLQ1jC1JIC&pg=PA222
What is true is that Bismarck did not originally favor taking Metz, but yielded to the military judgment of Moltke on that issue. (Indeed, there was a real dilemma here. Even a limited annexation--of the German-speaking areas of Alsace--would be enough to embitter France and make it dream of a war of revenge in the future. So to put Germany in the best possible position for such a war, it was thought necessary to take even more territory and thus embitter the French even more...)
"The annexation of Strasbourg and of Alsace appeared to him [Bismarck] necessary in order to enhance the security of southern Germany and make any French pressure on the South German states impossible." Hajo Holborn, *A History of Modern Germany: 1840-1945,* p. 222. https://books.google.com/books?id=Y4pLQ1jC1JIC&pg=PA222
What is true is that Bismarck did not originally favor taking Metz, but yielded to the military judgment of Moltke on that issue. (Indeed, there was a real dilemma here. Even a limited annexation--of the German-speaking areas of Alsace--would be enough to embitter France and make it dream of a war of revenge in the future. So to put Germany in the best possible position for such a war, it was thought necessary to take even more territory and thus embitter the French even more...)