16.1(b) How can a ruck form
Players are on their feet. At least one player must be in physical contact with an opponent. The ball must be on the ground. If the ball is off the ground for any reason, the ruck is not formed.
A ruck ends successfully when the ball leaves the ruck, or when the ball is on or over the goal line.
So in the case of that caterpillar the ruck was formed according to definition from law as member from each team were in physical contact on their feet. In law, there is only one successful way a ruck can end (stated above in law 16.6) and the ball did not leave the ruck and the law doesn't mention anything about opposition players leaving to end a ruck.
The most frustrating thing about everyone saying we need a law to stop it happening and how the IRB are looking at it, is that there is already a law for it!
Law 16.7 (b)
Before the referee blows the whistle for a scrum, the referee allows a reasonable amount of time for the ball to emerge, especially if either team is moving forward. [neither were] If the ruck stops moving, or if the referee decides that the ball will probably not emerge within a reasonable time, the referee must order a scrum.
If only referee's applied the laws they were given!
(I didn't think there was anything in the Tom incident, both players looking at the ball, just a collision, not a penalty or a card for me.)