> Overstressed vowels: A lot of the time, people are lazy about pronouncing certain vowels—they’re un-emphasized and neutral, and just sort of hang loosely in the middle of the mouth, making an “euh” sound, regardless of which vowel it actually is.

Here's a funny notion. Perhaps in modern American English (at least the midwest accent), perhaps many of the vowels have actually become schwas and it's only the historic nature of the orthography that makes us think they should be something else. There's no vowel that sounds like the schwa naturally, but it's clearly a "missing" letter in the modern alphabet since it's also the most common vowel sound in American English. So every other vowel ends up being pronounceable as a schwa in various contexts.

Strangely, most people who use the schwa vowel regularly make a specific point to pronounce other key marker vowels correctly in order to distinguish the words, the rest of the vowels are unimportant to distinguish.

Imagine how much better English orthography would be if we could just use an 'ə' everyplace an unstressed vowel existed.