The growth rate wouldn't slow to a crawl at ~30TH/s (and even turn negative) if the network was operating consistently at 50 TH/s over the last couple months.  A lifetime average asymptotically approaches the short term average.  In other words for it to flat line at 30 TH/s instead of 50 TH/s means the network isn't operating (finding blocks) like a 50 TH/s network.  You don't find the fact that shareholders own a 50 TH/s network which operates like a 30 TH/s network to be useful?  There is a difference between the lifetime average being 30 TH/s and still showing strong growth towards (but never reaching) 50 TH/s and one where the lifetime average stagnated at 30 TH/s and even dipped below its peak.

Please graph a set of points there are a significant number of recent data points at 50 anythings and the lifetime average does not aproach it.