The A7 series certainly fit well the situation, but what is the point of using the exact same bulky FF-optimized lenses if the initial goal is to significantly reduces the overall system size and weight? The camera body size will be for sure smaller but not the lenses, so on the whole the size and weight reduction will be only minimal.In my office bag I could carry my Mac Book Pro and Canon 5D MK III + 24-70 f/2.8 lens and a 16-35 f/2.8 lens ... but now I can carry 2 cameras (a7+a6000) and 5 lenses (10-18 + 24-70 + 70-200 + 55 + 85 with Metabones adapter) and a MBP in the same space. Lenses for the mirrorless cameras may not be as fast as the DSLR equivalents but they are a happy compromise.
Most people look at some camera/lens size comparison websites and come to incorrect conclusion that there isn't much of a difference in size, but one has to carry the mirrorless cameras and lenses around in bag to know the difference. What the mirrorless system does is reduce the overall weight and the space they occupy in your camera bag, plus you don't need to carry heavier tripods ... I now use Benro's MeFOTO Roadtrip tripod, (including ballhead) which fits inside my carry on luggage in the flight ... that's 2 mirrorless cameras, (1 full frame + 1 crop sensor), 5 lenses (with a FOV from 15mm - 300mm), 1 tripod with ball head, 2 speedlites and a Mac Book Pro as hand luggage in one single bag.
Only those who travel frequently can appreciate the flexibility and ease that such a setup provides ... also the resulting weight reduction only brings back the joy of being able to use the gear you want anywhere. Many photographers I know (including me) have carried lots of DSLR gear only to realize that sometimes the space and weight took away the fun factor of photography (bcoz on many occassions we didn't get to use the gear we carried), so we tried taking only a limited amount of gear only to feel guilty for not carrying a particular lens which would have been perfect for a given situation ... but the mirrorless system provides you with the luxury of being able to carry 2 cameras with half a dozen lenses along with a light weight tripod & a MBP as hand luggage ... now that is awesome. Yes, as of now the morrorless system cannot do everything what high end DSLRs or "L" quality lenses can do, nevertheless its a happy compromise, one which I am very happy with at the moment.