>>12
To add to your point, GMOs should function not as replacements for local edible flora entirely. If you look at what genetic modification did to rice, it created the strongest hardiest rice plant that can grow virtually anywhere vegetation is viable. The downside to this is the loss of biodiversity in rice - it is very hard to find anything but this ultra-durable rice grown, even in regions where a specific kind of rice could flourish. While the strength of the rice is formidable, that durability is mainly environmental; the consolidation of type means that if its armor is compromised then rice production itself is compromised.
inserting genes that make plants produce harmless pesticides found elsewhere in nature
This is a lingering concern even, as a proponent, I have for GMOs: it's very rare that humans try to introduce something from one location into one location for expected benefits and not cause worse problems. At least, with small animals and insects, our track record is considerably poor, even with exceptional planning.