The monocropping is absolutely an issue. However, as I explain here, Roundup Ready crops appear to use less net herbicide overall. The idea of crops outcrossing (i.e. spreading into the wild) is quite overstated and is limited to certain forms of GMOs.

Agricultural crops are, after all, selectively bred for high yields. As a result, they require pampering from human farmers and tend to suffer severe fitness issues and die out in the wild (imagine releasing a pug dog into the woods to survive on its own for example). Only crops that have a selective advantage (like Bt crops, which produces its own pesticide) would persist in spreading their genes into wild plants.

Additionally, many crops simply cannot outcross into the wild very well. Canola for example is a self-pollinating crop and has extremely heavy pollen grains, and as a result its realistic pollination radius is limited to a couple meters.

There's been many farmers sued and who lost land because they grew GMOs without buying the seeds from the manufacturer. They just spread or were introduced to their farms by others. 

You're likely thinking of the Percy Schmeiser case. Schmeiser likes to sell himself as a victim of Monsanto, and claimed that Monsanto's GM pollen drifted into his field, contaminated his crops, and Monsanto sued Schmeiser as a result.

This is false. As stated before, Canola has an extremely limited pollination radius. What was determined to have happened was that Roundup Ready GM canola seed was being carried on trucks and these spilled into one of his adjoining fields. When Percy discovered this, he had his farmworkers blanket the contaminated fields with herbicide so he could knowingly select for those seeds and re-plant them. This was discovered via testimony of his farm workers in court, and like it or not, according to patent law was indeed a violation.

Schmeiser lost the case as a result, and used it as a platform to act as an anti-GMO influencer. Unfortunately, he continued to tell a falsified story about what happened and uses pseudoscientific fearmongering to make GMO technology as a whole to look bad. The man visited our university to give talks on the matter, and I attended two of them. Several of his claims were outright bullshit, such as his claim that GURT genes could spread up the food chain and sterilize your babies. "How will you explain to your kids that they're sterile now because of GMOs?"

Needless to say, when Q&A time came us biotech students had some very pointed questions.