That's actually a possible theory, that they guilted themselves into not using such destructive force as their magic. Imagine what terrible damage Crono and Lucca could have done with Luminaire and Flare. It's also a point that Lucca spoke of (or at least wrote about) her regret over changing the timelines and damning the other timelines and the people along with them to the Darkness Beyond Time. As is known, in our own world and that of the Chrono series, time will fix itself. Events will always end up happening at some point. All that meddling with time does is avert it for a spell before it or something equivalent happens later. They may have defeated Lavos and prevented it in particular from destroying everything, but it would end anyway that millions upon millions would end up dying from some other catastrophe; perhaps an asteroid crashes to Earth, or, so God help us, another of Lavos's kind drops down. In that train of thought, pretty much everything they've done would only last for so long, and if htat's what they would come to believe, then it would prevent them from interfering. If the people of Guardia were to be slaughtered as they were, why not let the people, who would end up being killed in some other war in all likelihood, die then and at least be replaced with those who were developing, rather than have those who were so much more capable die later? It would certainly weigh heavily on all of their minds that what they did, for the most part, was all for nothing, and most likely make them succumb to despair, and essentially want to just get it over with.
This, of course, is all just speculation over the possibility of whether or not they would know, if not outright accept that certain things are inevitable. So they managed to restore a desert into a forest, but did it truly matter? No, it did not. Whether or not you do that particular sidequest, by the time Lavos springs from the earth it has all reverted back to desert sands again. Not quite a thousand years, at least, once the forest has been completed, and it's gone again. That sort of thinking is nearly as fatalistic as Schala's views on things, but it isn't as if they could make a true impact. Let it happen while things are still healthy enough to heal over rather than when it would be too late for said healing to do much good. On those premises, it would at least be more beneficial to let things take their course and let time march on again as it fixes what was done, and allow what was there to finish growing rather than have something happen later that could end up destroying any chance for the world to reach its full potential.
...Whew. I talked way too much there. Also, note that where I am, it's ten 'til three in the morning, so if I don't make too much sense, then I apologize.
