Is E=mc2 without Meaning?

As noted in the previous post the new 2019 SI definition of kilo kg as unit of mass is based on Einstein's 

• $E=mc^2$      (E)

stating equivalence of energy $E$ and mass $m$ mediated by the speed of light $c$, which is viewed to be signum of the progress from classical physics to modern physics of relativity and quantum mechanics. 

Endless speculations have been devoted to give meaning to equivalence of energy and mass apparently expressed by (E). But is it really possible to give a meaning or is (E) meaningless? 

Let us consider some basic facts, starting with units. Energy is expressed in Joules with 1 Joule = 1 Newtonmeter as the work performed by force of 1 Newton over a distance of 1 meter. Energy is here viewed as a capacity to do work measured in units of work = force x distance or 

• Joule = Newtonmeter.      (1) 

Next, mass $m$ is classically viewed as a measure of resistance to motion or inertial mass expressed in Newton's 2nd Law

• $m = \frac{F}{a}$

where $F$ is force and $a$ acceleration. When $F$ is gravitational force $m$ is referred to as gravitational mass which is equal to inertial mass with unit 

• $\frac{Newton\times second^2}{meter}$.     (2) 

We see that the units in (1) and (2) are very different and requires the coefficient $c^2$ in $\frac{meter^2}{second^2}$ to harmonise. 

There is classically no relation between energy as capacity to do work and inertial/gravitational mass. 

What Einstein did with $E=mc^2$ was to make energy "equivalent" to mass thus breaking completely with classical physics where there is no relation between energy and mass. This break is viewed to be the main heroic accomplishment of modern physics from the hands of Einstein. Paradoxically, this was never awarded any Nobel Prize, evidently because the Nobel Committee could not understand the meaning of (E).

Einstein thus connected two entirely different aspects of physics, namely work and inertial/gravitational mass, which have no connection whatsoever. No surprise that this caused a monumental confusion which opened modern physics to also other forms of confusion. 

To give perspective on the absurdity of $E=mc^2$ recall from the previous post that transforming 1 kg (of e.g. sea water) per second into energy is "equivalent" to a power of $10^{17}$ Watts, to be compared with the power of 1 nuclear reactor of $10^{9} $ Watts, thus equivalent to the combined power of 100 million nuclear reactors! Can we conclude that (E) lacks meaning?

Recall that $E=mc^2$ is the main result of Einstein's special theory of relativity, which is critically analysed in Many-Minds Relativity.

Recall further that Einstein uses the concept of "rest mass" as the mass of a body at rest (in some coordinate system or with respect to other bodies), carrying the idea that the kinetic energy of a body in motion will add to the "relativistic mass" of the body as being bigger than the "rest mass". If you do not find this confusing, you need to study the question in more depth.