"common sense says, that if you forcibly introduce and maintain a large number of new genes to the ecosystem, it has potential to unbalance things"
no, it doesn't. Also the genes in plants change all the time.
Common sense says ma will never step on the moon.
Common sense, isn't.
No, common sense is, until (scientifically, or whatever) shown to not be. Furthermore, when common sense says, common sense isn't, then it's common sense to assume the worst case scenario, until shown otherwise by a more reliable method.
"Natural evolution can only keep up at the rate of natural evolution,"
than's been shown false.
You realize what you say makes no sense? What I said was tautology. Perhaps you meant "under pressure, natural selection can work surprisingly fast"? Yes, but there's generally a cost of a high extinction rate, and then diversification of those species which did not die. This is not a very desirable, especially when there's a big mass extinction underway, species disappearing very very rapidly (in geologic/evolutionary time scale).
"in very small steps and under constant pressure of natural selection."
not always. if a gap appear in the eco system, say all the dinosaurs wiped out, other creatures will evolve to take advantage of the new space pretty quickly.
Either you don't care about humans, or you're intentionally trying to muddle up human and evolutionary time scales. Recovery takes millions of years. We've either spread to the space or gone the way of the dodo long before any natural recovery has happened. Also, for recovery to happen fast, there needs to be lack of competition. This implies that there are no humans competing with the new species. Doesn't sound very good to me.
"and we can manipulate natural selection "
which has gone really well, actually. Billions are fed that would have otherwise dies becasue of that.
In other words, we've used our ability to adapt to increase our population, steadily approaching the carrying capacity of earth. We'll probably be in trouble long before we have the technology to supply everybody with food grown by nuclear energy (instead of solar energy and fossil fuels for fertilizers and machines, as now happens) or whatever. There's only so much net output available from solar energy.
We've increased human population to current billions (and counting) with the cost of ongoing mass extinction, decrease of biodiversity, and overusing natural resources (overfishing, burning forestes to get a few seasons worth of crops before burning the next piece, etc). Furher cost, which I hope we will not have to pay, but which seems inevitable, is that these billion will die, when we are at the practical limit of carrying capacity, and then there is a reduction in our food production (it could be for political reasons, or because fossil fuel based fertilizers become too expensive, or because climate shifts and large areas of arable land become too dry, or many other things).