Sometimes the only way to stop the try is to shortarm the attacker, that's not permitted either [yes yes, i know it's slightly more extreme].
We just had this discussion; where the ball was grounded
despite the defender using his arm on the only part he could on the diving defender.
If we go with a momentum interpretation of the game, I'd argue that a defender is within his rights to stop an attacking player in the act of grounding the ball, as long as there is no foul play.
By the letter of the law, yes, this could technically be considered a PT. Or not, see later.
Fair contest? There would be NO contest if the consequence were not only a PT but the obligatory YC for the defender for a PK offence in the zone.
Continuity of play? In a game where a TMO is available, the referee has the luxury of checking if he is correct as to grounding. Imagine that red 8 HAD grounded the ball, their supporters would be "gutted". But the TMO is there to make sure there are no "howlers", similar to DRS in cricket. In any other game, it is either scrum 5 or try where grounded. It is
not within the remit of the TMO to mention that is ITMOHO a PT, even less so when that isn't vaguely the question being asked.
Safety? Not an issue, unlike the case of the shortarm.
As to law 14.2, the ball carrier is not "lying on the ground". Lying, similar to sitting and standing, is a verb which to me evokes a certain lack of dynamism, as opposed to such verbs as diving, sliding, walking and running. In other terms, the first derivative of the position of the player as projected onto the line of touch is (near) zero.
Back to fair contest: not giving up when defending is to me just as important as atractive rugby with ball in hand. Brilliant attempt at a try by red! Spirited defence by octarine making the play even better. Correct use of the TMO.
Materiality: I found it attractive rugby. Clearly material, given 5+ or perhaps even an almost certain 7 points available. All that is material though is WAS THE BALL GROUNDED. No it wasn't. Red gets another chance from 5m out, so their brilliant play does not go completely unrewarded.
Question: if you thought this SHOULD have been a PT, do you also agree it MUST be a YC?