My rationale for Cheika's lock switching:
Coming out of the RWC2015 we had the following five locks in or around the Wallaby setup:
+ Kane Douglas: the most promising of the Wallabies locks in RWC2015, but people still questioned his selection in the squad. Did his knee in the final and hasn't been the same player since.
+ Rob Simmons: a workhorse with zero impact. Non-Reds fans have been questioning his Wallaby position since day dot, and even Reds fans have jumped on the bandwagon these days.
+ Sam Carter: Rob Simmons lite. Non-Brumbies fans have questioned his Wallaby position since day dot.
+ Dean Mumm: Sam Carter lite. Everybody, bar his mother, have questioned his Wallaby position since day dot.
+ Will Skelton: the anti Carter/Mumm/Simmons. High impact but low workrate. Hasn't been able to play the full 80 throughout his career, wasn't a reliable jumper etc.
The most experienced was Simmons on 52 tests, followed by Mumm on 36, Douglas on 15 and Skelton on 11 (can't find Sam Carters 2015 stats).
For me, and many posters here, it was patently obvious that the second row was the weakest part of our squad, and given that the second row is one of the most important partnerships on the field, we had to find other solutions.
This was made even more clear given Dean Mumm's retirement, Kane Douglas' knee injury and Will Skelton's departure overseas.
Where are we now?
In my mind, the 2nd row hierarchy is as follows:
Adam Coleman - 16 tests.
Rory Arnold - 15 tests.
Rob Simmons - 60 tests.
Izack Rodda - 3 tests.
Lukhan Tui - 1 test.
Kane Douglas - 23 tests.
Sam Carter - 16 tests.
Quite clearly, Cheika has brought through Coleman, Arnold, Rodda and Tui (with 35 caps between them) extensively whilst still giving Simmons and Douglas enough game time to be familiar with the squad environment.
And what do we need? I'd suggest that by RWC2019 we need four locks (+ perhaps a utility lock / blindside flanker), all of whom we are comfortable with being in the starting XV in case of injury.
And who are they? I'd suggest that as of now, Coleman is confirmed and Simmons is pencilled in as a workhorse and role player who won't let the team down (and can be used heavily against Minnows).
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.