I suggest you look at the laws of the game, as well as the IRB recommendations to referees regarding dangerous tackles.
Because it's the law, it's therefore correct and should be applied in unwavering terms and with no room for interpretation? Excellent.
TV pundits are TV pundits, whether ex players or not. Their role is to either be anodyne or controversial. It's absolutely ridiculous for them to say "... oh but it's a World Cup semi-final, he's ruined the game by sending him off..." It doesn't matter if it's an under 12s game or a World Cup final, a dangerous tackle is a dangerous tackle. The ONLY person who "ruined" the game was Warburton by being so stupid. At my rugby club EVERYONE watching was of one opinion... red card!
Fine, but can you make your mind up as to which argument you're using, please? Is it the 'no-one who has played the game can argue with it' one or the 'my mates at the rugby club agree with me' one? I'd be inclined to trust the chaps who have played the game at that level rather than your mates.
Let me ask you this... during the Spain v Holland World Cup final, who ruined the game? The Dutch players by adopting the tactics they did, or the ref for trying in vain to keep as many of them on the pitch as he could? If 3 had been sent off (which was justified) would the ref have been blamed by the pundits for "ruining the game"?
Nigel de Jong studding someone in the chest is so far removed from what's acceptable in football that the correct decision was a red card. Warburton's tackle was so marginal in between the 'legal' tackle and the 'spear' that a straight red was a bad decision. Before you bring up the fact that the IRB has banned Warburton for three weeks, that's irrelevant. Yellow card offences are capable of incurring longer bans even where the referee has correctly said that a red card wasn't sufficient.
Ignore TV pundits. Listen to current players in clubs up and down the country. The consensus on the rugby message boards was that it was a sending off offence.
Again, I'd prefer to listen to people who have played the game at that level rather than rugby club know-it-alls who spend most of their time drinking eachother's piss in pursuance of 'edgy banter'.