Psykerduck
When you dig around and come across all manner of stuff?Leaving aside your extreme reluctance to attempting any kind of quantification of said magic properties, have you perchance missed that now the light of Magnus shines over France?
That the water of France will gain the local properties just by becoming part of the local water cycle?
That the local properties will pass into the plants of France as they use that magical water and magical sunlight to fuel their growth?
That those very properties will also enter the citizens of France when they eat food made from those plants and drink that water?
Humoring the notion that the essential elements of Tamriel are radically different because of a magic x factor, the fact that said x factor is provided by sunlight means that France will gain the aforementioned x factor in exceedingly short order.
Not decades. Not years. Not even seasons. You should rather think weeks, if not days.
As it happens, sunlight is not a scarce resource easily kept out of French hands.