Clinging to "oral transmission" is like any other clinging… I believe most traditions acknowledge the possibility of Pratyekabuddha,
which would show that most traditions do not limit Awakening to oral transmission (or even any
Impermanence applies to Buddhists too: there is nothing to justify that "it always will be" a particular way.
Regarding your argument "If the Buddha wanted to reduce what he taught to writing one would think he would have, given all his other abilities," +Bill Schwartz
, one could argue he did:
Writing does not matter so much, what matters is reliable preservation and transmission (which writing is a good tool for, but not the only tool…). By organising a saṃgha a lot more than any other Śramaṇa, the Buddha did
organise preservation and transmission. His agenda seemed quite clearly to have been about spreading and preserving the teachings. And if one relies on oral-based preservation, one needs a stable and large community [it takes a lot of time to learn sutras "by heart" if one cannot use a written version to memorise them, and so for transmission one needs regular meetings —rainy seasons,— over many years, and a large community so that each monk only has to memorise a small amount for posterity]. We know the saṃgha was divided in Sūtras-holders and Vinaya-holders, and that the Sūtras-holders each memorised only a few sūtras (not the whole collection: DN, MN, AN, etc.).