that's the point though --
[LAWS](d) Tactical advantage means freedom for the non-offending team to play the ball as they wish.[/LAWS]
normally a team will wish to attack, so if they haven't got the ability to do this then no advantage is gained.
but in the circumstances of the OP we are told that the team do NOT wish to attack, but instead what they wish to do is ---
so once they form a tidy ruck, and any pressure they did have is relieved then they have got the freedom to play the ball as they wish (which is to recycle it a few more times until the whistle goes)...
You have to make a call one way or another, you can't wait any longer as they are never going to achieve territory, or an attacking option, as they aren't trying to.
So while the Law is the same in the 79th minute, what the team is trying to do has changed.
you could think territorially and call no adv gained, scrum
or you could think tactically and take the view that they have gained the freedom to play the ball exactly as they wish, and have therefore gained a tactical advantage as defined: adv over.
So you think that the team is
"playing playing the ball as they wish" when they run a series of "pick and drives" in the last minute, but they are NOT
"playing the ball as they wish" when they did so in the 27th minute? If that is true, why were they doing it?
Let me outline for you a perfectly plausible scenario (one that is often repeated at all levels of rugby) that will help you to understand why I strongly object to your approach to managing advantage.
Same situation as the OP, BUT the Red team is 6 points ahead,
and needs one try to claim a bonus point and deny their opponents a close loss bonus point. Red start to pick and drive under advantage; are they...
► Simply trying to wind down the clock to end the game, happy with the win?
► Trying to gain some ground to draw a PK so they can kick a goal and at least deny Blue a BP?
► Trying to score the try to get a BP themselves
and deny Blue a BP?
Maybe they aren't even aware of the BP situation, or are not thinking of it, but how are you going to know that? What has happened to your context now?
As I said earlier, both teams have every right to expect consistency; to expect you to apply and manage advantage in exactly the same way in the 79th minute that you did in every other minute of the game. If you applied advantage differently in the 79th minute to the way you applied it to the rest of the game, you have not been consistent.
You have spent the first 75 minutes not calling advantage over until the ball is well clear, a couple of passes wide of the ruck, then suddenly in the 79th minute, you are calling advantage over after a pick and go. If I am your MO, you are in for a discussion about consistency in your rulings.
Trying to second guess what teams are doing or trying to do is fraught with danger. The safest path is to play exactly what is in front of you and remain consistent in the way you manage all aspects and phases of the game, including advantage.