Responsive design focuses on delivering the content based on screen resolution (pixels).
As screen resolution sizes of mobiles and tablets increase (tablets are now desktop monitor screen resolution), the effectiveness of responsive design becomes obsolete. If a tablet has desktop monitor screen resolution but is physically still 10", the website will be too 'small' to interact with and responsive design won't help.
Shouldn't responsive design focus on a screen ratio (take in account screen resolution AND physical device size)?
Responsive design focuses on delivering the content based on screen resolution (pixels).
Not quite; that's not the spirit of responsive design, it's just the current best method of implementation. If devices start reporting better stats about what they physically are, those stats could be used insteadResponsive
also means taking target devices and consumation habits into account for how content is served, consumed and layed out.