The Phillips D was desgned to throttle the pass once the O built a quick lead. It was supposed to be strong on the outside, albeit vulnerable to the run up the middle - and got gashed by running teams, especially in tight games. Without a quick strike offense it was doomed.
Rapp was drafted to a cheering crowd here on the board. When he became a square peg in a round hole, his accolades fell off. Simply put, he can't extend coverage against speedier backs or receivers. You have to get there first - and once there, have to keep pace. Rapp, despite being a thumper close to the line and being a heady-enough football player, lacks the physical traits (speed, athleticism) to play more territory deeper as demanded in the quarters system.
For twenty-plus years with a rabbit ears antenna and no available cable to pick up network broadcasting of "regional" games I have been consigned to watching incessant Seahawks and Broncos games, Sunday-in and Sunday-out. Rams sightings, until they became playoff contenders, were rare.
During that time I watched plenty of Legion of Boom D, with Cam Chancellor playing near the line and Earl Thomas deep. (After being educated as to zone/quarters by JY, I made the remark that Thomas would have been an ideal Rams safety. JY corrected me saying that he played in a different system. I knew that but saw enough of Thomas to believe that he could have played anywhere.)
Rapp would thrive, if not as another Cam Chancellor, in such a system. Thump the runner, thump a WR in limited coverage responsibilities,and just for grins blitz every now and then, play close to the LOS while somebody else, a ball-hawk with speed and awareness, covers deep.
You need 2 better-than-good cornerbacks to play that system. Seahawk opponents would game-plan to pick away at whoever was playing opposite Sherman. The Seahawk D during that era had strength enough up the middle to squelch the run, and could pressure the QB - the "off" corner could be picked on when there wasn't enough pressure to disrupt the QB.
There was more than the Legion of Boom defensive backfield to make that system go. There was a middle linebacker who helped stuff the run and could cover, sideline-to-sideline. There were pass rushers - maybe not Hall of Famers, but guys who could get there.
Morris comes in, Phillips leaves. Morris, I'm led to believe, is told by McVay what type of a defense he is supposed to run. However that went down, Raheem is supposed to make square pegs fit round holes. Adapt the players, regardless their strengths, to fit the scheme - not adapt the scheme to fit the skills of the players.
Flash forward: The preferred McVay/Morris/Rams D, regardless how it was chosen and implemented, has to go searching for edge rushers to make the system work. It works well enough to win a championship with Von Miller. We know what happened to the secondry without him. Nobody looked especially good.
Rapp, for the last couple of years, improves his coverage skills noticeably, but still stuck out as a square peg in a round hole, being asked to do perform beyond his skill set. He did get a couple of picks.
Not his fault, or the fault of those who picked him in the draft to do something else. I wish him well, and wish the Rams well in getting safety help that matches their scheme.