If the root cause is not delamination reducing capacitance, then it has to be to do with attenuation of the signals, and so most likely caps related. Or possibly gradual decline in the conductance of the traces leading to the screen edge connections to each matrix line. I haven't seen inside one of those, but if the traces on the connecting cable fan out from the centre, the traces to the corners will be longer, and attentuation would be seen there first.

Just speculation...

Rick