That’s possible, I guess… but I’m not sure it entirely fits. In The Cerulean Storm he starts the transformation anyway.
And Rajaat was the one who taught everybody magic in the first place. If he was worried about their magical capabilities, it’d have been easier to keep magic to himself.
Also, it would seem that if that was the only reason, he’d only need their civilizations to be destroyed, not the entire species (there’d be no need to hunt down every hunter-gatherer band or remote isolated village).
I think there has to be more to it - either in Rajaat’s own warped psychology and motivations, or in the magic involved. For some reason, Rajaat at least thought (at the time, 3500 years ago) that extinction/genocide was a necessary prerequisite - but apparently no longer thinks so at the time of The Cerulean Storm.
And something turned Rajaat from mutant-pyreen to the gigantic shadow/elemental being we see in The Cerulean Storm. His imprisonment in the Hollow beyond the Black is clearly involved, at least for the shadow aspects, but OTOH I’m sure the imprisonment wasn’t meant to give him enormous elemental powers.
Someone suggested that the Cleansing Wars were actually a mass gathering of life force, which might work.
Another possibility is that Rajaat was doing his own advanced being transformation while the Cleansing Wars were going on - some unique type not described in the rules, that leads to the “clouds for flesh” water/rain based form we see.
In either model, Rajaat at the time of empowering the Champions wasn’t powerful enough to turn the world back to the Blue Age, but was that powerful by the time of his imprisonment.
…
But … the use of the Dark Lens (a ridiculously powerful magical & psionic amplifier) as a physical weapon, and the imprisonment instead of killing of Andropinis (especially since he specifically says “a thousand years” of imprisonment - Rajaat is way older than that, so that’s not effectively-forever like it would be for us), really makes me think Rajaat has some kind of limiter on directly using his magic to kill.