An intuitive emc^2 relation was proposed before Einstein on the notion of charged particle's energy mission in EM radiation, although the energy was correlate with charge not mass.

Einstein's derivation was warmly welcome because it solidified previews intuition of great scientists (tolver preston, 1875) and thus pushed towards unifying physics (j.j thomson 1881, Oliver Heavyside 1889).

But given that the premise of the proof were now the strange 4-spacetime, it also strengthened that case. To the point that it might as well be regarded its biggest (and SR's) validation evidence i.e theoretical