How are the Genetically Modified seeds technically impacting farming in the United States?

The barriers are marketing. You can raise non GMO grains (but these would not be Organic crops) but you  won't be able to sell them easily, if at all because almost all the buyers buy GMO grains only these days. Also it is getting quite hard to find non GMO seed for certain crops

And before GMO's farmers were not growing Organically, they were growing conventionally using synthetic fertilizers and pesticides, generally had no soil fertility plan, did not practice much crop rotation (usually corn, beans,corn, beans but generally not a 4 or 5 year crop rotation).

So the reason they don't go back to raising organic grain crops is because they never did that sort of thing/never were organic growers

First off, Monsanto isn't the only company that produces GMO crops and holds patents on them.  As for the patents, it's no different than the patent Microsoft holds on Windows or Apple holds on Mac operating systems.  They all spent years and lots of money developing their products and deserve to profit from their work.

Not that many farmers really lament the patents seed companies hold on their seeds (except that everyone would want a cheaper price). Buit if they really wanted cheaper seed, it's out there. Most farmers voluntarily choose more expensive, patented seed because it's more productive, and yields more. Also a big reason for buying GMO seed is that it's herbicide resistant. But really, there's nothing stopping farmers from going back to older varieties of seeds. Marketing isn't really an issue, grain elevators don't care what variety your corn (or whatever crop) is, in fact some of them would prefer seeds without certain traits, due to international trade restrictions, but if you have less grain to sell, you're going to get less money.

Most farmers are not organic farmers, so they wouldn't be "going back" to raising organic products. In order to go organic, they'd have to convert, and there are several obstacles to doing so.

The two biggest ones, in my opinion, are education - farming (organic or conventional) is a complex specialty, and if you're specialized in one method or another, you have a whole learning curve to learn another way of producing. And most people are not good at thinking "outside the box" and reconsidering how they do things.

The other - major- obstacle to going organic is that there's a three-year transition period for most crops. So if you're a conventional grower, you would have to switch to lower-yielding organic methods, but sell your crops to low-priced conventional markets for three years before you can sell to organic markets and get the higher price. So although organic crops can be - in some cases - more lucrative, most growers aren't willing or able to endure the three-year transition period.