You must also consider people who are not native speakers. I'm sometimes appalled by the complexity of the letters I get in Germany, especially when they are related to immigration.
> So, the solution is to dumb everything down to the least common denominator?
Essentially, yes.
Using words most people are familiar with will afford a lot of people nearly instantaneous cognition [0] of the information you are trying to convey with your writing.
This applies to both ends of the reading spectrum: highly educated people who do a lot of reading than most; and poorly educated people who probably had to drop out of school to make ends meet.
It's not about dumbing things down, since you convey the same information and solve the same problems, just more efficiently. It's about being understood by the users you are meant to serve.
