The improved reliability, range and accuracy of fire weapons reduced the effectiveness of old-style shock combat methods. Battlefield saber charges by large mass groups of cavalry became harder to manage and successfully execute.
Make no mistake, your post is broadly accurate on the increasing difficulty of cavalry charges in general (indeed in the Indian Mutiny the British regulars could maintain a half-mile "no go" zone around them where cavalry movement was highly dangerous when within line of sight), but it's almost irrelevant to the ACW because the weapons (and the weapons training to use the rifles effectively) simply are not there.
I'll also note that von Bulow's death ride was actually quite effective - yes, a lot of his men died, but nothing else could have tied up such a large number of French troops for so long as the chaos caused by the cavalry charge. This makes it an effective weapon, if one you can't actually reuse - there are several places in the ACW where the ability to fire a cavalry unit at an entrenched line and be pretty sure they'd make contact and disrupt the position would have been totally invaluable and swayed the course of the entire campaign (or indeed the war).
If you have adequate cavalry support in proportion to the size of your army, the loss of 420 cavalrymen (out of 800) for that kind of disruption is cheap at the price.
ED: on checking the charge I was referring to was New Market in 1864. The Texans still had smoothbores though...