Football Outsiders is definitely not perfect by any means but in my opinion it's strange that you bring up DeShon Elliott as an example to prove your point.
He was a sixth round pick in last year's draft who ended up on injured reserve. I'm not sure he would have had any impact if he stayed healthy though.
As far as I can tell they didn't assume the Cardinals had 14 starting caliber offensive linemen for their metric but just mentioned Arizona had to use that many last season.
The reasons I cited Elliott are as follows:
1) Baltimore had the lowest AGL of 29.7 which seemed too low to encompass the full range of injuries a team will suffer from the start of OTAs to the end of the season. Actually, those injuries could reach back to the 2017 season where a guy had a very serious injury, spent that year on IR, and started 2018 on PUP or again on IR.
2) Then, Elliott was the first player named in the list I found of 7 Ravens who went to PUP and IR at the start of the season. He was the first and only player I looked at to determine a particular player's games lost. I saw just now that one of those 7 Ravens was a 4th. round wide receiver. Is he or any of the many other players in a similar situation a better example? Maybe.
So, having seen only FO's Part I when I first cited Elliott, the very first and only guy I looked at was sufficient to demonstrate FO did not include all injuries in their tallies. For all I know, had Elliott been uninjured and had the opportunity to domonstrate his worth through a full camp and preaseason he might have been cut. Among the many players across the league who started the season on PUP or IR there are probably better examples where the case to exclude them from AGL consideration is more debateable. Of course we'd have a hard time debating some of those without actually knowing if they were excluded or not without a simple extrapolation Elliott happened to provide.
Once I saw Part II it became evident (I think) that FO was counting only starters and "rotational players". They don't explicity state the methodology; we glean from the examples that those are the players they are considering which goes to one of my points in the Elliott vs. Cockrell. It isn't until we get deep into Part II we find:
"The Carolina Panthers (47.4) had the most defensive back AGL we have ever measured. Free-agent signees
Ross Cockrell and
Da'Norris Searcy missed the entire season with a broken leg and 14 games with a concussion, respectively.
Both would have started if healthy."
Now we see the level of subjectivity and the assumptions involved in the data in Part I. While the particular assumptions might seem obvious in the Elliott vs. Cockrell comparison, how many across the league might not be so simple? And why do we have to so deep into the piece to have to make suppositions?
I'm just now thinking about the Packers last season, which begs a couple of questions. Did they count Kumerow in the WR missed games as a possible starter or rotational player at some point in the season had he not been injured? Don't know. Probably not. That's a guess. I'd have to dig through all the receivers who were on the roster and examine the weekly injury reports to attempt some kind of determination. I should not have to do that.
Just now I got to thinking about Davis and his missed games to start the season. Did they count him as a "rotational player" as the presumed KO and punt returner? Then it struck me that they have no AGL category for special teams. You have to go all the way to the end of Part II to see that, and then that fact might have escaped one's attention on first reading. There's no accounting at all for kickers and punters so I would guess, and it is just a guess, they would not count players whose primary role is returner. While that is a reasoned guess, it is a guess nonetheless, that Davis is not counted in WR AGL. To figure that out for a certainty you'd have to do a game-by-game dive into the Packers player-by-player 17.3 WR AGL to get an idea. Somebody else can bother with that, but they shouldn't have to.
As for Arizona's 47.8 AGL and 14 starters on the O-Line, I see no reason to believe they did not include missed games by players 6-9 in that number, the first line backups who missed games leading to players 10-14 having to take the field. Player 9 might be a third stringer dragged off the PS or the street who had no business starting in the NFL. Or maybe he wasn't injured and just benched for another second or third stringer brought in from somewhere? 5 full season equivalents lost on the OL is a ton. Why not believe, given FO's specific mention of these facts, that they did not include players 6-9 missed games if they were in fact injured? Again, it would take a deep dive to reckon who is included and who is not in that AGL.
So, the point is not whether or not this FO analysis is "perfect". I don't expect perfection. What I do expect is transparency and a statement of methodology, not led down some primrose path with a table of raw numbers only to have to surmise, glean, assume and conjecture what those numbers are actually telling me with, at the end, some critical unanswered questions.
What we need here is a Part III, a table of all the players for all the teams who were counted in AGL. Why not produce that unless they wish to avoid scrutiny of the subjective elements of the analysis? Is there one? I'm not going on a wild goose chase for something that might not exist. That list should be linked in Part II if there was any interest on FO's part in any kind of "peer review".
This is a habit of FO. Lots of data, unstated framing of the terms of the discussion, opaque methodologies, black boxes.