We grow around 80% of the veg we consume, including across winter and the hungry gap (having a freezer makes preserving much easier than it would be for our ancestors)
Our sprout and cauliflower crop failed this winter, our sprouting brocoli is stunted but hopefully will come through, we've lost 50% of the broad beans we planted in autumn, luckily we get a second chance in spring with those
I would hate to have to only rely on the foods we produce, there is great comfort in knowing if the sprouts crop fail we can just buy them
But yes we eat seasonally and its not all turnips in winter but I will be the first to admit as the freezer stores run low in the hungry gap, my DH isn't examining the asparagus patch every day for the sheer joy of fresh asparagus (well not only for that) it's also for a change in veg!! And I also admit to sometimes buying tomatoes in february because who doesn't want a change from beans, root veg and brassicas!
If you are advocating for the eating of seasonal veg because its more environmentally friendly because it hasn't been transported as far, then that's great. If you are advocating for the eating of seasonal veg because you think we shouldn't be importing veg then you are in for some very lean eating, even in summer