untouchability came with cleanliness. I am so sad that people dont see through things. Even today it prevails. Can you embrace the munciplaty/corporation drainage cleaner? I am 100% confident you wont.
Brahmins - did vedic rights Vishnavas - did business Kshatriyas - did protection Shudras - Did social work
It might have so happened that a person too keen on clenliness started to keep away the sweaty ones. And this got transfered from generations without second thought. I agree that it is mistake. But the utmost reason for un-touchability is cleanliness. Who wont respect you if you are clean and tidy?
Unfortunately people like Mahatma Gandhi killed the symptoms, and not disease. He didnt ask shudras to stay clean, but asked upper caste to adopt them. Even he lacked vision in many places.