Food is a national security issue. You cannot become dependent on other countries.
Those subsidies are just enacted to serve entrenched farming interests, usually because rural voters have a disproportionate say in national politics.
If a country was designing a food subsidies policy that was truly concerned about national security they'd do things like maintain a strategic buffer of cheap canned goods, and certainly wouldn't be subsidizing expensive to produce calories like French cheese other animal products which are highly inefficient per produced calorie.
The question as I understand it is whether the surplus production/exports are worth not scaling slightly back on the extensive farming.
Either diversifying production, letting a part of the area to fallow or splitting large "strictly for profit" farms into distinct units would have many benefits ecologically and possibly even help revitalise the rural towns.
You'd instead at the very least just have them drink the milk directly, but more efficiently either consume the corn the cows are eating, or grow something in the grass fields the cows are using that people could eat directly.
Then there's the unrelated question of whether these subsidies aren't causing a net harm on the environment in France. I don't know about that, I'm just pointing out that the argument for agricultural subsidies due to national security concerns doesn't hold up if it's being used to defend anything resembling current subsidies.
That's before we even get to discussing how implausible that argument is to begin with. OK, maybe France and other EU countries would need to strategically make sure they don't become reliant on the likes of China, Russia etc. for food imports.
But it makes no sense these days that France would need to be concerned that they couldn't just largely import food from Spain and Italy, who have onerous EU-imposed production quotas due to French agricultural lobbying, even though it's more efficient to produce the food there in many cases.