So what would be the effect on the ecosystem if a species with built-in insecticides got out?
Which is not to say that there aren't any risks at all. But plants and insects are already locked in a millions of years long battle/cooperation. And look how quickly insects have developed resistances to our modern pesticides. Adaption to something new appears to take years to decades, not centuries or longer.
Some insects. And you're very correct that the insect-insecticide battle has been going on for a while - that doesn't mean giving one side a huge leg-up won't have consequences. Genetic engineering can accomplish millennia of evolution in a single generation - the two are not the same.
But genetic reshuffling, recombination, and horizontal gene transfer has been going on en masse in the microbiota we are surround by for billions of years. I think once we start considering introducing truly novel genes, we will have a case for greater caution and reserve. But so far, all projects are of the 'transfer this already existent gene to this other organism', and I think that is a less risky proposition overall.
E.g. how can you know that transferring a gene from maize to petunias is still "sufficiently close" to the level of variation seen in ancient natural processes and evolution?
(Genuine question but also genuine worry about this.)
A few tobacco plants in isolation are going to be protected from insects easily developing a resistance to their natural defenses. We've been growing tobacco plants in massively concentrated fields for 400 years in this country. Nicotine is still a useful insecticide, just as Penicillin is still useful antibiotic.