We can see a similar drift toward complexity in the written language by looking at text messaging in the modern day. The old system was that e.g. "before" was written "before". Now it might be written that way, or it might be written "b4". The second form is easier for the scribe, but requires a lot more knowledge on the part of the reader. The shorthand, which makes things difficult for the partially-educated (such as foreigners) is a result of mass literacy. If text messaging had been restricted to formal elite-to-elite communications, they would have gone on writing "before" the same way they'd been doing it all along.

I suspect something similar contributed to the trend you describe in Akkadian texts; with Akkadian in decline, the only people using it would probably have been more familiar with it than average. And that familiarity would let them use it in ways that wouldn't have made sense to a merchant's wife trying to send a letter to her husband abroad.