I disagree, I don't want a relative metric. You're saying scale factor neatly encapsulates viewing distance, eyesight, preference, but compared to what? Scale is meaningless if I don't have a reference point. If I have two different size monitors you have now created a metric where a scale of 2x means something completely different. So to get things look the same I either have to manually calculate DPI or I have to manually try and error until it looks right. Same thing if I change monitors, I now have to try until I get the desired scale, while if I had DPI I would not have to change a thing.

> It is easier to have human say how big/small they want things to be than have gazillion flags for individual attributes and then some complicated heuristics to deduce the scale.

I don't understand why I need gazillion flags, I just set desired DPI (instead of scale). But an absolute metric is almost always better than a relative metric, especially if the relative point is device dependent.