Actually, best as my non-phsysicist brain can understand it -- we DON'T have gravity all figured out. Sure, we define it and we can make great predictive statements about it (in general), such as the formula G*M1*M2/R**2.
Fine. Yet, far as I know, we have yet to find proof of a SINGLE "graviton". (If Einstein is correct, and gravity is a space/time phenomenon, then why the HELL particles have the FIRST thing to do with the actual mechanism of gravity is completely beyond me). But again, I'm just a stupid layman, lacking formal math skills beyond a few semesters of college calculus, etc.
So all I'm saying here is -- yes, there are obviously no simple perpetual motion machines, and it gets really tiresome when some simpleton claims to have found one and then argues that everyone ELSE is an idiot if they don't agree with him/her.
OTOH, lest we get too smug, let's remember we're just relatively hairless apes who have noticed a few things like a proximal relationship between a force and mass. That hardly makes any of us candidates to "know what is really going on".
Except, of course for VM. His mystic powers are the key to EVERYTHING, which is why he's in my lonely "ignore button" collection.
Given the track record of the perma-doomer blogs, I wouldn't bet a fast crash doomer's money on their predictions.