Let humidity drop to 10-20% and only bump it to 50-60% when about to shed (success)

[SDA]

What exactly is the evidence that it is harming the animals? I've noticed no side effects whatsoever.

Do you now have a hygrometer? If you let your cage stay at 10-15% your snake would not thrive. The first major sign would be dented eyes. What you would not see is kidney distress leading to anything from failure to kidney stones. Constipation is a major concern as it can lead to GI tract issues. One obvious sign is lengthwise skin folds in the snake which are a clear sign the snake's body is trying desperately to conserve water.

Notice I don't mention repository infection? You have to have an infection source to cause infection. Extremely low humidity in a snake not designed to conserve moisture when breathing will cause repository problems but it won't automatically cause a RI. That being said your snake will have issues maintaining proper humid conditions in their lungs at such low humidity.

I am not talking a few days of low humidity but if you keep it constantly like that for months (10%), these signs and symptoms will occur. The longer you let this foolishness go, the worse it will get. What I am getting at is keeping it like that will destroy their immune system and overall health and if not now, down the road you are going to have a problem.

Stop being stubborn and take care of that snake or get one that is more adapted to a desert environment.