Anything above 24 mL is considered dangerous if you are using refillable tanks. Very few people even bother to that high because it is much too strong. The most I could handle was 12mL, but that was on very low-powered devices.
You go down because you go for more power, partly. When I started, you could only get up to maybe 10w. These days, people are going up to 100, but I'm pretty sure they are not using any nicotine for that.
You also just don't need as much over time. Nicotine also over-ride the flavor.
Most vaping devices (mods) produce much denser vapor and doing 50 mg/ml nicotine with them would not be pleasant even for an experienced smoker. People who use vaping to quit smoking gradually decrease nicotine concentration to wean themselves off nicotine.
You go down because you go for more power, partly. When I started, you could only get up to maybe 10w. These days, people are going up to 100, but I'm pretty sure they are not using any nicotine for that.
You also just don't need as much over time. Nicotine also over-ride the flavor.