I think the selection of Tom Youngs ahead of Hibbard shows that the Lions aren't going to try to just out-muscle the Aussies. Hibbard is the better scrummager and he would have fit that type of game better. What Youngs offers is a more athletic carrying option and a huge work rate in and around the ruck (and a better lineout throw judging on the Lions tour alone). I see this as a positive step by Gatland to play a more rounded , fast tempo game.
We may not quite see the best out of Adam in the scrums without Hibbard, but it will probably mean there will be almost no let-up in intensity when the front-row replacements come on. I wouldn't be surprised is the front row as a unit is replaced as a whole, with Vunipola, Hibbard and Cole expected to come on and really pile on the pressure, in terms of scrummaging and carrying, late on.
For this reason, I also understand the selection of Corbisiero ahead of Vunipola. Vunipola has done everything right, but his impact from the bench could really sway this game late on. We really don't want to slip-up in the final few minutes like Wales have done so many times over the past couple of years against Aus. Lydiate also fits into this. If we are leading heading into the final quarter, expect Lydiate to come on with a sole purpose, stop Austrailia in their tracks, because they will up the tempo and will apply pressure.
I'm not quite sure I agree with the Lydiate selection though. If we don't experience any backrow injuries in the game, it will prove a good selection, but if Warburton or Heaslip get injured early on, it could really mess up our balance in the backrow. It's a bit of a gamble imo, especially when we have the versatility of SOB and possibly Faletau (could easily play on the blindside), and the brilliance of Tipuric in the squad.