There are two sides to that one. And two components to the other side.
Component 1) About 40 years ago, I was riding out CA 205 from the bay area to Manteca. Two lanes. I'm in the right lane. A car is pacing me a few car length back in the left lane. After about 5 miles of this, the driver pulls ahead of me, then drops back next to me, then starts to change lanes. I was riding my '74 R75/6. The one with the air horn. Compressor the size of a beer can. Two trumpets larger than the compressor
I hit the horn, and the driver was back in her lane faster than anything I'd ever seen before. You see, she thought she'd cut of a big rig, and that was a lot more motivation to shape up and fly right than "just" cutting off a motorcycle.
Component 2) A lot of drivers either can't hear a motorcycle horn, or simply don't care.
Summary:
Based on my experience, I WANT them to think I'm a rig. I honk because they are ALREADY heading toward me, so their most likely instinctive reaction will be to go the other way - would you really start to change lanes, think you're fouling a rig or train by the lane change, and KEEP changing lanes? Sorry, but I absolutely do not agree with you assessment of the situation. Both intellectually, and by decades of experience riding in traffic.
Motorcycle horns are far to quiet. The horn on your bike needs to be loud, and it needs in cause either a major pucker or a hershey squirt. I'm not a fan of loud pipes, but I am absolutely a fan of loud horns.