It should be illegal to refuse warranty for hardware, because of software modifications and no, I can't think of any way software could cause hardware failure, even massive overclocking should pretty much end with thermal shutdown and no drama whatsoever. Knox is not bad idea, but it should be included on separate business line of phones only. There are many versions of GS4 like mini, active etc. why not come out with secure GS4 business with Knox and leave consumer phones alone? They even pushed Knox on my 1yr old GS3, which I couldn't care less since it's out of warranty anyway, but it's just another useless bloatware and one more reason to root to get rid of it.
To me it is insane that I can't fully back up my phone without rooting and rooting could void my warranty.