I considered this a little bit today... well, there is a workaround explanation if we take the idea that today is a Golden Age and life span is actually increasing, not decreasing. Though it is a little bit clumsy. If we take into account commentarial explanation to one sutta in SN about life spans, then in our case there appears an explanation that we shouldn't take direct approach to this sutta prophecy. How is that? In DN Buddha says that in the past the life span was 80.000 years, then 70.000, ect... to 250. And then it lowered to 100. After that Buddha uses future time, and says it will be 10 years. So the explanation is this: when he spoke about 100 years life span, he missed some past period when the life-span decreased from 250 to 100 and then to 10 and then raised back again to 100. So actually Buddha appeared in the world when "the era of increasing" started (where we are now). But - he tells us that in the future life span will be 10 years, not 250. So the idea is that it will, but only after the peak of 80.000 ,) So at the present time life span is increasing, and in some future time it will be 80.000, but then it will drop to 10 again. And from there it will reach 80.000 again (and it is here when Buddha Metteya appears).
80.000 > 250 > 100 > 10 > 100 (here we are) > 250 > 500... 1000... 80.000 > ...1000... 250.. 10 (1st prophecy) > 100 ... 250 ...1000... 80.000 (Metteya prophecy).
But, if we take the idea that today is a Dark Age, then like this:
80.000 > 10000... 250 > 100 (here we are) > 10 (1st prophecy) > 250 > 1000 ... > 80.000 (Metteya)