It's not pace by itself that threatens defenses given how handy the touchline is for defense. Trimble and RK both have 2 tries this tournament. Both Trimble and DK have been very good in close contact, DK in particular has a knack of squirming out of tackles he has no right to get out of. This does threaten defenses it takes defenders out of the game leaving them numbers down for the next phase.
Thinking Launchbury "easily" caught DK is also completely twisting something to suit you point you give no context to it. DK had eaten up metres pretty quickly up to that point and had Brown in front of him. So he cut in so support would be near him and he wouldn't get knocked into touch. Launchbury had the angle on him and got an ankle tap on him as he came inside. An ankle tap, from someone who's 6'6" with long arms diving to get him. Hardly getting caught easily now is it? We also recycled the ball well from it. It was the intelligent thing to do.
Yes clearly McFadden has lost so much pace since then.
Seriously is that all you've got? I could have posted his try against NZ from 2012 if you wanted for an example. 132 metres and 3 clean breaks in the last test match he start, that's not bad "pow".
http://youtu.be/uD7LnCAjbbU
http://youtu.be/rrqAoJeXDrM
http://youtu.be/Czp579ILGSU
http://youtu.be/wutdXKURISg
http://youtu.be/kbuFsNeOq8A
http://youtu.be/H7UJ1mnSFgM
None of them will make top 10 lists of fastest players but they don't need to, that's not our gameplan. None of them will let us down due to not having enough pace either. Please let me know if any of those clips are too old.