You see: you have to add these words "immediately playable" into the law that are not there in order to justify the position that, I agree/accept, most if not all referees take.
There is no immediately playable exception to the outcome of a collapsed maul provided for in the laws.
To play as if there is one is to deprive the laws of the effect they would otherwise have: for one thing it would actually open up the contest for possession because teams would be much more wary of forming a maul if the knew that it could never become a ruck. It is axiomatic that there is much more open contest for possession at a ruck than there is at a maul.
I don't mean to be unnecessarily contrarian, but I think you are essentially wrong about this.
Let me quote the relevant law:
17.5 SUCCESSFUL END TO A MAUL
A maul ends successfully when :
• the ball or a player with the ball leaves the maul
• the ball is on the ground
• the ball is on or over the goal line.
The key to interpreting this situation is in recognising that if the ball goes to ground the maul is over successfully. (17.5).
Then what happens? Would you at that point immediately allow any player from any direction to immediately go and play the ball? I don't, and no-one else does either. Instead we say that the offside lines remain, and that players who wish to play the ball must come from behind their own last man's foot. It isn't a maul (by 17.6 the maul is over), but equally it isn't open play either.
Is it a ruck?
16 DEFINITIONS
A ruck is a phase of play where one or more players from each team, who are on
their feet, in physical contact, close around the ball on the ground. Open play has
ended.
If the ball is on the ground, and there are at least 1 or more players from each team on their feet, I can see no reason in
the laws why a ruck cannot be formed.
What happens when the maul goes to ground but the ball is not on the ground?
17.6 UNSUCCESSFUL END TO A MAUL
...
(b) A maul ends unsuccessfully if the ball becomes unplayable or collapses (not as a result of
foul play) and a scrum is ordered.
...
(f) When the ball in a maul becomes unplayable, the referee does not allow prolonged
wrestling for it. A scrum is ordered.
(g) If the ball carrier in a maul goes to ground, including being on one or both knees or sitting,
the referee orders a scrum unless the ball is immediately available.
There is a conflict between 17.6(b) and 17.6(g). On the one hand if a maul becomes unplayable or collapses, it ends unsuccessfully, but on the other hand, if a ball carrier in a maul the maul ends unsuccessfully if the ball is not immediately available. The way it is usually resolved is that once a maul goes to ground, the ball has to be immediately available or its a turnover. It doesn't have to be used immediately, but it does have to be available to be used immediately. As you point out below, there is no obligation on the non-ball carrying team to roll away or to let the ball go. If the halfback has to have a prolonged wrestle for it, its a turnover.
Where I do take slight issue though is what happens if the ball is on the ground. The maul is successfully over (17.6), but I don't think we have open play yet either. I think it is a defendable position to take that if you don't have your hands on the ball when the maul goes to ground, and the ball is on the ground, you are not allowed to subsequently play it if (a) you are off your feet or (b) you do not come from behind the hindmost foot. I genuinely don't know of any other sensible interpretation of this mess of the laws than the one that I have given and the one that hopefully is generally applied.
But there's another insidious consequence of pretending that a collapsed maul has/can or will become a ruck: and that's the ref calling for the players of the team that did not take the ball into the maul being directed by the ref to roll away. There is no warrant for that - it is not a tackle and it is not a ruck so you guys have no basis for telling players from the team who did not carry the ball into the maul to roll away from the ensuing pile up - but, to man and woman, you all do it.
This "interpretation" changes the whole dynamic!
Exactly. I agree that if the ball is off the ground no-one should be told to roll anywhere because the ref should have blown the whistle for an unsuccessful end to the maul. It happens far too often for my liking too and I must say I don't say this myself, I just follow 17.6(f) and just blow the whistle.
Where the ball is on the ground and we have a successful end to the maul, the ref can, I think, ask a player who is off their feet and offside to not interfere with the ball. Rugby is a game to be played on your feet.
As to the final point - this doesn't happen at the elite level much but you have refs calling mauls when they aren't. So its general play - cant be offside..you'll never see a garden variety ref cotton on to that difference in my experience.
Maybe, but aside from questions direction of entry or perhaps calling for players to get back onside a fraction early I doubt its much of an issue. 9 times out of 10 it becomes a maul pretty quickly and the other time the ball gets stripped and passed away.