Like I said: I realize that edge players will always have better stats on paper than inside defenders, we can stop going back and forth agreeing on this.
"I'd imagine most teams want to run outside because if they can get the edge, they usually only have to beat a man in the secondary for a big gain." Exactly my point, an edge defender is on an island tasked with covering lots of yardage whereas inside defenders (especially in the 4-3) only have to handle a small area between them and the next guy. And I'd argue that the rest of that paragraph can be said about both, if the edge is locked down the runner can cut it inside (which would pad the DT's stats and lets not act as if Donald hasn't had fantastic edge defenders on that Rams defensive front, which is also an argument pertaining to his pass rush prowess, he's never had a supporting cast anywhere near as bad as Mack, in fact his supporting cast since being drafted has arguably been the best in the league...)
If a DT gets pressure then the runner can bounce it outside which would easily benefit and pad the OLB's stats but why is the same not true for DTs when edge players get immediate pressure? usually if any DT gets immediate pressure on a run play, barring a missed tackle, they are able to get the runner no problem. Only in rare cases are you dealing with a runner able to make the move quick enough in that short time period to get away by bouncing outside, in such a case you are dealing with an elite runner and it doesn't matter what position you are talking about with instant pressure.
And pertaining to that last sentence, that's not necessarily true at all, Donald had more stops than most OLBs, including Miller, not Mack but most.