Correct. But there are states further east than the ones you name, with more precipitation and thus less need to mine water.
I thought I linked to it earlier: Water From The Great Lakes. It covers the engineering/physics challenges of moving water from the Great Lakes to the arid West, the legal issues, the ecological and ethical issues, at least one link to a prior thread, the Aral Sea disaster, a number of posters admitting to peeing in Lake Michigan, and quite a bit about the California Central Valley starting with the post by @not_alice in post number 64.
Those “corn and wheat fields” can grow vegetables, though. Lots and lots of them. They used to grow them back when long-distance transportation wasn’t as developed or efficient as it is today. They still grow them today even if in smaller numbers. While no comparable patch of that area is going to be as productive as the CV because they get fewer sunny days and have an actual winter, those new fields won’t require major irrigation works, and there’s a lot more land area than the CV so after a period of adjustment (which yes, will not be fun) equal quantities of food can be produced. Vegetables and fruit might become more seasonal than they currently are, but it wouldn’t be the end of the world.
You’re assertion they can’t be used for growing fruit is incorrect. They can’t grow citrus fruit. The “stone” fruits - apples, pears, cherries, etc. all grow quite fine in places like Michigan and Ohio and so forth. We have grapes - both table and wine - growing in Indiana and Michigan and probably elsewhere east of the Mississippi that I’m not aware of. We won’t be growing avocados, either, probably but I’m sure the Mexican cartels will be happy to sell them to us. If we want American grown citrus maybe we should start helping Puerto Rico to rebuild their orchards, which we should have started doing back in 2017 but, well, politics. Come to think of it, we might need to boost the Florida citrus growers as well, at least until the state goes underwater but that’s a few decades off so we have time to plan for it (although, based on past history, I expect humans will ignore the problem until it’s too late).
Replace the almond orchards with dragonfruit - it’s native to the American Southwest and grows just fine in arid regions, it’s a freakin’ cactus after all. It’s what we should be planting in dessert regions.
And, in fact, there IS a lot of farmland that is not growing food right now because agriculture moved west. What you don’t get is individual mega-farms the size of a small country. Yes, adjustments would not be trivial but it could be done.
Cheap MEAT might soon be a thing of the past. Inexpensive meat such as we currently enjoy is actually a historical anomaly (admittedly, an enjoyable one for most). If we gave up meat (not likely, but I’m giving you a hypothetical) that would free up about 80-90% of that farmland for other uses.
Yes, there are annual variation in yields. Also, which fruits and vegees are you buying? Some cost more than others. Seasonal costs less than non-seasonal. Out of season frozen might be less expensive and are just as good for most purposes. Admittedly, frozen lettuce for salad is not a thing. So it’s a good thing that there are people trying to develop greenhouse technology allowing things like fresh lettuce year round, and some of them are operating in urban areas, not traditional farmlands. They use less water, less land, don’t need pesticides, and can operate on the roof of a building or urban “brownsites”, and produce at a regular rate year round.
I’m not advocating plowing up the CV tomorrow. It does have a role in US agriculture, a very large one at present. However, it is NOT essential. A pain in the ass to replace, true, but it could be done. If there is no more water for the CV, or some other disaster takes place, we may have to replace it.