The problem with physical sizing is that 1cm on a large monitor will still look different to 1cm on a 5" mobile phone, because of the different viewing distance

The problem with physical sizing is that 1cm on a large monitor will still look different to 1cm on a 5" mobile phone, because of the different viewing distance. If you had the same physical size, a browser window may be 20cm across on the monitor, but on the phone you'd see a few cm and a scrollbar. Anyway, it's pretty ridiculous once you start using units without regard to their definitions, like points and pixels. You'll just end up with a big mess. If something different is wanted, then it's time to define something new.