If we still had Ellis, I assume he would still be a ~200 carry back. That would be consistent with how he was used here, which included years we had pass-first QB (Benkert) and a run-first QB (Perkins).
It would also be consistent with how Anae used his tailbacks at BYU -- one bellcow getting a ton of carries. He had Curtis Brown then Unga, then DiLuigi's high-carry year in 2010. Anae was gone in 2011-12, but got back to it with freshman Jamaal Williams in 2013 and 2014 when Williams was healthy. You reference 2015 in another post, but Williams missed that year, and Hill went down for the season early, so I doubt 2015 is reflective of what Anae really wanted to do.
Operating from that assumption, my take is the staff just hasn't thought we had a tailback as good as Ellis since he left, else that player would be getting bell cow carries. Fairly sobering take, considering I always viewed Ellis as 'solid but unspectacular.'