I know that it tends to be only applied when the ball is dead but the law just says "A player must not intentionally waste time" without any mention of circumstances.
At the back of a ruck is different as was pointed out. The opposition aren't free to attack the ball in any meaningful way unlike in your 7s example. If there is no law stopping it why not just stand there for 40 minutes if you are ahead? Unsporting?
In a theoretical sense you are right, but in reality, this would never happen. Long before the 30 minutes was up, the opposition would commit whatever numbers were necessary to push their opponents off the ball or force them to play it.
Remember also that the Maul Law with the 5 second limit is not a FK for time wasting, its simply a scrum turnover.
Personally, I support the Cambridge Law trial idea of a "use it" call then five seconds to do so, but that is not current law, and referees must not make it up as they go along.
If a referee were to award a FK for time-wasting in this situation, it would be a definite "gotcha". He would have to sell it to the affected team, and more importantly, he will (IMO) have a hard time selling it to his assessor.