If someone adds 5 riding to their build, that's 5 less WM, 5 less IF, 5 less shield they could have. That's 2 more attributes, plus a skill.
You act like 5 riding is something anyone can just throw into their build.
Most people are min/maxers. But you may be shocked to realize that although most cavalry don't go pure cavalry builds (because higher riding skill than the horse requires is negligible compared to native, or 1.5 years ago in c-rpg). But you could make an 18/24 pure lancer cav build at level 30. With 6 PS, 8 Riding, and 3 athletics. You could also go 21/21 with 7 PS, 7 Riding and 3 athletics. Or you could go more balanced and go 18/18 at level 30 (1 IF, 6 PS, 4 Shield, 6 Riding, 6 Athletics, and 6 WM) and have 118 1h, and 119 polearms.
Even within cavalry you can choose to trade off certain skills to maximize others, but most people just go pure hybrid because having more riding skill doesn't really help much for extra speed and maneuverability on the horses. Before they nerfed riding skill on horses, you had people who were going 9 and 10 riding because they wanted to be pure cavalry builds. Now there's no point in doing that because you wont' be riding much faster than 6 riding on a courser compared to 9 riding.
Every nerf has unintended consequences, and that is just one of the 100's that have happened because of a nerf. You nerf something, and people will adapt their character and play styles.
I say (for the most part) leave the game alone and don't nerf shit.