To base the whole philosophy of nature upon light is a baseless assumption.

The Michelson-Morley and analogous experiments show that within the limits of our inexactitude of observation the velocity of light is an approximation to the critical velocity 'c' which expresses the relation between our space and time units.

It is provable that the assumption as to light by which these experiments and the influence of the gravitational field on the light-rays are explained is deducible _as an approximation_ from the equations of the electromagnetic field.

This completely disposes of any necessity for differentiating light from other physical phenomena as possessing any peculiar fundamental character.
It is to be observed that the measurement of extended nature by means of extended objects is meaningless apart from some observed fact of simultaneity inherent in nature and not merely a play of thought.
Otherwise there is no meaning to the concept of one presentation of your extended measuring rod AB.

Why not AB' where B' is the end B five minutes later?
Measurement presupposes for its possibility nature as a simultaneity, and an observed object present then and present now.
In other words, measurement of extended nature requires some inherent character in nature affording a rule of presentation of events.
Furthermore congruence cannot be defined by the permanence of the measuring rod.