But these aren't any worse than what is already used, though. Again, these aren't problems that are "GMO" problems, they're problems that are agricultural in general.
My point is, these problems may be exacerbated by the widespread adoption of GMO crops. Kinda like if you see a forest fire, well, it's already burning, so I'll just leave these open buckets of kerosene here and walk away because the forest fire isn't the kerosene manufacturer's problem.
That would depend on whether or not you have to purchase the seeds fresh every year or if the plants are sterile. If they're treated like the usual crops, biodiversity would build up in the same way that it builds up through normal crop breeding...through successive generations. I don't see this as a problem except when it's tied to the intellectual property laws (which right now of course it is - I've made it clear though that I find those problematic).
I can't speak to every GMO manufacturer, but Monsanto has farmers sign contracts to purchase new seeds each year. Keeps their profit margins high. This has figured prominently in several court cases, including the infamous Monsanto Canada v. Schmeiser, which I highly recommend people read about if they are interested in some of the shenanigans that surround the implementation of these technologies.
But let's assume you are getting a better contract than most farmers are, for just a second. There's a balancing act here where whatever benefit you acquire from the initial adoption of GMO (i.e. crop yields, ease of pesticide or herbicide use, etc.) needs to be balanced against wiping out the biodiversity of your crop for at least a few generations and potentially having severe consequences in the process. It's not a clear-cut decision, and the impact of a few farmers doing it vs. everyone doing it can also have consequences.