C represents the speed limit of the universe rather than the speed of light

[_PJ_]

First of all, one unwritten universal law is 'everything in this universe has a limit', so does the speed of anything. But the question is 'how do we know maximum speed limit is c?' It is just 'one theory' that tells us c is the highest speed of things. A theory.

If we use E=mc², and assume c is the highest speed of any object, doesn't it also mean 'the rest energy of mass m is twice its maximum kinetic energy?'

You're assuming rest mass is equivalent to obserrved mass.

 

[DrGreg]

If we use E=mc², and assume c is the highest speed of any object, doesn't it also mean 'the rest energy of mass m is twice its maximum kinetic energy?'

No. You are assuming the formula for kinetic energy is ½mv². But it isn't. That's an approximation that's valid only at low speeds. The accurate formula is

$$\left( \frac{1}{\sqrt{1-\frac{v^2}{c^2}}} - 1\right)mc^2$$​

There is no upper limit on kinetic energy.

(I'm using the convention that the vast majority of physicists follow nowadays, that "mass" m means "rest mass".)