Re: Pakistan vs South Africa 1st Test Match 2013
We can’t get anyone’s “replacement” all the time considering most of these players were “hand-picked” for their talent and did not necessarily make it to top based on performance in domestic or under-19. Our players selection has problems that’s why domestic performers were not picked as much in past. Things are improving I believe so the selection is based on part domestic performance, part captain’s friendships/relationships in domestic players, selectors favorites, “recommendations” (aka sifarshis).
Other reason for not having a good pool of bench is that we don’t get exposure to international cricket as much, no international teams coming to Pakistan which allows domestic players’ top performers to play against quality players in side matches. But still 
we do have bench players who can step up.
As far as your other point is considered about not every team performing every where that has always been the case. Only in case of a pure dominant eras (WI, Aus) we had a team who will perform anywhere other than that every team had good region of performance and not-so-good/bad region of performance (sub-continent vs Eng/Aus/SA/NZ etc). Currently there is no completely dominating team but one will emerge sooner or later.