Yes, but prices have also been rising over that time. I expect that if prices had remained constant our usages would have gone up exponentially (equivalently, s

Yes, but prices have also been rising over that time. I expect that if prices had remained constant our usages would have gone up exponentially (equivalently, supply would have had to increase exponentially to keep prices constant). If we have low energy usage over the next 4 billion years, but only because the price keeps going up, then it has not 'last us 4 billion years'. The price going up is precisely what 'not lasting us' looks like!