The intersection of people that care about high dpi laptop screens, but not about high dpi external screens is very, very small. Also, if you are in that group, why wouldn't you just turn down the resolution on the laptop or even turn it off (since you connected a low dpi external monitor, you don't seem to care about the additional sharpness of high dpi, right?)?

High dpi is important as is low dpi (for a while still at least), mixed dpi is an edge case even if you disagree.