A thought has recently occurred to me. There are possibly more devourers out there than are known about but going around glassing each planet that develops magical life is slow and rather unrealistic way to go around doing the job.

Is it possible that the devourers are trying to simply erase the thuamic field instead so that there is no magic at all.

Might sound absurd to try but wouldn’t it be something they’d do to achieve their goal especially if the ponies of earth become a problem for them?

While not out of the question, they could desire this, considering they don't have any sense of self-preservation as robots; erasing the thaumic field means destabilizing the entire universe. Removing the thaumic field would be a universe-ending event where the universe would go through a very rapid big crunch and another big bang as it reset.

However, the new universe would develop a thaumic field, and there would be no more Devourers to combat whatever arises in this new universe since they'd not be there after the big crunch.

What they'd love to be able to do is get rid of all the super-concentrated thaumic matter, which would make the re-emergence of a proper universe (as we understand it) highly improbable (though one would eventually emerge after enough big bangs and big crunches, though that could take an untold number of universes to accomplish), but they don't have the capability to destroy super-concentrated thaumic matter.