Where are we going?
In 2017 we have the following tests remaining: Argentina, NZ, Japan, Wales, England, Scotland (+ a Barbarians game).
= 6 games.
In 2018: Rugby Champion Fixtures (6), Bledisloe 3 (1), Mid Year Internationals (3) and a Spring Tour (4).
= 14 games.
In 2019: Mid Year Internationals (3), Shortened Rugby Championship (3), Bledisloe 2 (1).
= 7 games.
Therefore, assuming 3 locks per match day squad, we have 54 caps to be allocated.
Assuming Coleman plays 75% of available games (due to injury), he will receive 14 caps (0.75*18, rounded up).
Assuming Arnold plays 50% of available games (due to injury and form inconsistency), he will receive 9 caps.
Assuming one of Tui and Rodda is in every match day 23, they will both receive 9 caps.
This leaves 13 games to be distributed among Simmons, Douglas and Carter, with each being hypothetically given 6, 4 and 4.
Where does that hypothetically leave us?
The Starting Second Row:
Adam Coleman - 30 caps.
Rory Arnold - 24 caps.
The Reserves:
Rob Simmons - 66 caps.
Izack Rodda - 12 caps.
The Second Row / Flanker Hybrid:
Lukhan Tui - 10 caps.
The Fringe Dependables:
Kane Douglas - 27 caps.
Sam Carter - 20 caps.
Summary
Of course, this is riddled with assumptions and overly simplistic analysis, but in my mind, we will reach RWC2019 with a clear idea of:
+ Who our two best second rowers are.
+ One immensely experienced reserve (Simmons).
+ Two players with enough experience to not be considered "green" (Tui and Rodda) (and whom I think will get more experience than assumed above).
+ Two last resort, dependable backups (Douglas and Carter).
If we just ran with Coleman, Arnold, Simmons every game until RWC2019 we'd end up with:
+ A more experienced starting second row (Arnold, Coleman).
+ One immensely experienced reserve (Simmons).
+ Two last resort backups who wouldn't have had a minute of test match rugby in three years (Douglas, Carter).
+ A whole bunch of untried, untested, undeveloped players (Tui, Rodda, etc.)
Imagine if Coleman or Arnold, or Coleman AND Arnold, went down in the lead up to the comp.
We'd either be running on with two dependable, but average players (Simmons, Douglas, Carter) or two completely untried but high potential players (Tui, Rodda etc.) in the most high pressure environment in International Rugby.
Obviously I've really spelled it out but Cheika's rotation policy has built a lot of depth in the second row, and for a good reason IMO.