The shrimp example is kind of strange. Like you said, it's an extremely common character, and not a difficult one either. But beyond that, if you look at it he got the radical, 虫, correct. The phonetic element, 下, is a fundamental character that I doubt anyone forgets to write.
It just seems like such an odd outlier example. Like talking about a friend that spells "been" as "bin." I'm sure it could happen, but it's not indicative of a broader trend.
The story was reported by Victor Mair, though, who is extremely opposed to using characters and often exaggerates the issues with them.
Personally, I've seen a lot of Chinese people's written notes, and I don't think I've ever seen them resort to pinyin, even among people that didn't go to college. I just asked a few Chinese friends about this, and they told me they never resort to pinyin either.
