There isn't a magical threshold where things flip from "harmful" to "not harmful" and it doesn't matter how far above the harmful line you are. Reducing your exposure to lead by 50% annually will have big positive health impacts even if it's still too high.

Actually there are limits for how much lead is considered harmful in your system. OSHA has exposure limits that indicate amounts per hour, and those amounts decrease the longer you're exposed. It takes a long time for lead to leave your system and prolonged exposure at levels higher than your system can process will result in a net gain or build up.

If you're above the acceptable limit, and you don't reduce exposure to below that limit, then there's no benefit.

OSHA picking an acceptable limit does not mean that the harm when below that limit is zero, or that harm does not increase as you go higher above that limit. Being exposed to ten times the limit is worse than being exposed to twice the limit.