Einstein Obsessed by the Speed of Light

Einstein founded his Special Theory of Relativity SR on two Postulates: 

1. The speed of light (denoted by c) is the same in all inertial systems.
2. Physical laws literally take the same form in all inertial systems.  

We have seen that since 2019 Postulate 1 is simply an agreement to define the length unit of meter in terms of the distance traveled by a light signal over a certain time interval, or equivalently an agreement to define the length unit of light-second as the distance traveled by light during 1 second. This is the new SI standard to be valid in all inertial coordinates system moving at different velocities. In other words, the unit of length in each coordinate system is simply adjusted so as to make the speed of light = c. 

The real question is to what extent different observers using different inertial systems, can agree. They agree on the speed of light but everything else is up to comparison.

Maybe this was understood by Einstein as an anticipation of the 2019 standard, or Einstein did not understand the difference between an agreement to use a certain standard as compared to actual physics not asking for any agreement as a physical fact. The fact that that the Sun rises every morning is not the result of an agreement but simply a physical fact.

In any case Einstein elevated the speed of light to have a fundamental significance. Einstein supported this idea by a claim that no physical effect can propagate faster than light (in vacuum), which gave light a special prominence in particular through the "equivalence of energy $E$ and mass $m$ mediated by  $E=mc^2$.  

But what is then the significance of the speed of light? Would the World be different if the speed of light was only half of current standard? Would the World be different if the speed of sound was only half of current estimate? What is the physical meaning of $E=mc^2$, if any at all?

It appears that Einstein in his 1905 patent office became obsessed with an idea that the speed of light $c$ has a universal significance, which he supported in "thought experiments" imagining himself riding on the crest of a light wave at the speed of light.  

We experience sound and light reaching our ears and eyes, but we rarely ask about the speed of sound or light, since it has no significance except as a nuisance in communication over large distances. 

Making something insignificant to something completely fundamental, will ruin any theory. Einstein was a master of such delusions.