There are few problems with this affair evolving into the 100YW.
1st, legalistic, as a result of the
Mad War (
la Guerre Folle) Anne's father agreed to the condition that his daughters were not to marry without the approval of the King of France (see what happened to her marriage to Maximilian Hapsburg regardless Austrian diplomatic protests to everyone including the Pope).
2nd, military, in the terms of the comparative military and economic strengths situation in 1480's was quite different from one in the XIV or early XV century. The balance changed into the French favor and English did not have a tactical edge which they enjoyed most of the 100YW (and lost by the end of it). Not to mention that England had been exhausted by the Wars of the Roses and hardly could afford a prolonged war. It is highly questionable if at that time England could conduct a war against France: when Edward IV landed in Calais in 1475 and found that Charles Bold is not coming he was more than eager to start negotiations (Louis XI, in his usual style, found that it will be cheaper to pay him off than to fight a war; the payoff was seemingly easy for France but big enough for Edward to "recoup his finances"). As the 1st of the Italian Wars demonstrated, France could raise an army of approximately 30K (Charles had 25,000 French troops and 8,000 Swiss) with the biggest and most modern artillery park of the time. Phillip de Comnines gave some high assessment of the Ed's landing force but his numbers were based upon a hearsay: he never saw these troops.
3rd, political, at least in OTL, the Franco-Breton war of 1488 - 91 resulted in the French occupation of the Duchy (and Anne's engagement to Charles VIII, notwithstanding her "marriage" to Max Hapsburg). IIRC, most of the Breton nobility switched on the French side even before the war was over and why would not they if the marriage agreement was guaranteeing
union of Brittany and France with Brittany preserving de facto independence with a guaranteed separate succession if there will be more than one child (female child or 2nd male child would inherit the duchy)? In OTL, it was inherited by Anne's only surviving daughter who became Queen of France by marrying Francis I. There was no serious reason for them to side with the English and to make the duchy a theater of the prolonged war and no excessively good memories about the English presence in the area.