The pack would be different. Hartley was throwing to Attwood, Kruis, and Haskell for three games.
a) He was doing it poorly. Our lineout at the start of the 6N was not good enough.
b) They changed the pack to include Lawes and Parling at lock by the end of the 6N - what makes you think they'd have changed back to the big guys if Hartley had still been available?
Since we've brought in Youngs, we've gone into full lineout-mode with Parling, Lawes, and Wood - dropping our best lock in the process. I'd also love to have a look at where England are now throwing to, because I'm not sure Youngs ever threw it past the middle jumper against Wales or Fiji. Which both ruins the point of having an attacking back line and is compounded by having a much weaker maul.
Since we're in lineout mode our scrum has been very poor, and only Marler's rampant cheating made it any good versus Wales. Not surprising when the back five are about three to four stone lighter than they were at the start of the Six Nations.
Can you remember how our scrum performed at the end of the 6N with Parling and Lawes in the second row? It's a lot better than you might think.
Launchbury really wasn't very good in the warm-ups and hasn't weakened our line-out when he has come on as a sub.
No, we're not throwing to the back very much, but then it wasn't very advisable in the 6N either. And I'd agree our maul has gone downhill, but then past results show the coaching staff don't care.
The only issue I haven't touched on is Wood vs Haskell. Maybe they'd have done that one different, but they do like Wood.
And this is why I don't think having Hartley would have greatly changed the make-up of the pack. Our lineout was bad with him. All of the changes are changes Lancaster likes to make anyway. Attwood didn't even make the squad.
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I'm also not sure it's accurate to describe our tight five as mobile. They're light and fit, with a good work rate, but they miss several attributes. They're handling is poor, they're not particularly quick, individual carrying is very weak, and -worst of all- they're awful at clearing rucks. I'd say we only had a couple of tight five players (Launchbury and Lawes basically) who could be described as genuinely mobile, and even then they have flaws.
Even if we had more, there just isn't the depth in English rugby (yet) to sustain that style. Three injuries in the Six Nations (Launchbury, Lawes, and Wood) and we were forced back to a very slow, strong pack playing an ill-suited gameplan. I think we'd be far better off starting with a strong, slow base and look to add the likes of L+L as bonuses, rather than as a basis. Once the U20s forwards grow up we then look the transition the national side over a few seasons. For the mean time, start slow and speed up in 2018-19. (At which point the backline should hopefully be experienced and cohesive enough to play that faster game.)
My two cents, anyway. I know most of that is subjective conjecture, but I hope I got the point across.
I can't agree with this and I think you're expecting a bit too much to be called mobile in the tight five. I don't think you have to be both a rucking and carrying monster to count. Very few players are both.
LHP - Marler has very good hands and is not slow. Vunipola has very good hands and carries very strongly (wouldn't swear to his pace). Corbs is quick, Mullan is quick.
Hooker - Youngs fits all of your criteria and so does LCD. Shame about the throwing. George really gets around the park and so does Taylor from what I've seen of him. Even Hartley gets around, although not all that effectively.
THP - Well, ok. In fairness, this is for the fattest of the fat. Cole... nope. Wilson... surprising turn of pace, but has shown some poor hands at times. Broookes is quite quick for a big lad mind and Henry Thomas was always disturbingly mobile for a THP, don't know if he's kept that.
Lock - L&L go without saying. Parling, for me, just fits into the lower end of the boundary although he's not very noticeable. Attwood blatantly isn't. Kruis though, Kruis has a bit of gas. And looking outside the squad, Itoje immediately jumps to mind. Kitchener has pace. Slater isn't slow although I'm not sure he's especially quick. If Josh Beaumont ends up at lock, he is quick.
We have the fastest tight five in the Six Nations if we want to pick them and their handling skills are pretty decent before we even consider the recent crop of U20s - and I think if you doubt the current guys you should doubt the up and comers because a lot of the guys I've named looked a bit rock star at U20. Ruck technique leaves something to be desired, but they're English rugby players, of course it does. Carrying? Really good carriers in the forwards are bloody rare, unfortunately. A pack with three is doing well - a front row of Vunipola George Brookes would be rude.
I'd just like to add three things to that:
a) I've named most of the forwards in contention anyway. If our best set-piece tight five doesn't come from those guys, we're probably screwed. There's a few guys who'd be jettisoned in a mobile tight five who'd improve our tight five, but a pack reliant on Attwood is just as vulnerable to injury as one reliant on Launchbury - and more so, given the lack of cover. If Marler-George-Brookes-Launchbury-Kitchener doesn't do it for you at set piece, not a lot will.
b) If people want to go set-piece orientated, then what are you doing about defence? The more static pack was part of a team that conceded a lot of tries in the Six Nations. The more vulnerable your big guys are to getting picked off, then the more tries they'll leak, and the narrower you have to defend, conceding more tries out wide as well.
c) If people want to go set-piece orientated, who's supporting our back three? Are we expecting the back row to do it single-handed? Or are we simply going to go for ten man rugby?
Final note - I don't care if we don't have a Plan B. Wales don't. Ireland don't. You can go a long, long way on executing Plan A well enough - and I think Plan B is a luxury in terms of coming up with a Plan A equal to taking on those teams and winning the 6N.