I think there's definitely a connection between the general quality of handwriting and the economics of letter-sending.
Before railroads, and in the tree-stump filled wilderness of the Eastern US, frequently before ROADS in the late 1700s and early 1800s letters were expensive and took forever to get where they were going, so people put a lot of time and effort into their letter writing.
I've seen a magazine article from the 1870s about how railroads and "instant" delivery of communication were killing the art of letter writing and how with the transcontinental railroad complete and post getting less expensive, letters were getting shorter and much less formal. It read a lot like modern laments.
Once you get desktop publishing, and any individual has powers over reproduced documents similar to those of a small printing outfit, the value of a handwritten manuscript has just fallen through the floor. Combine that with the way America has shed a lot of traditional class signifiers since the 30s (the strange world where a man making $80,000 a year and a man making $8,000,000 a year both shoot to be living similar "middle class" existences that differ only in scale.) Baroque handwriting has vanished in kind of the same way that compulsory Ancient Greek at upper-crust colleges has. And for more or less the same reasons.