I'd agree that there are swings and roundabouts and you will always have weak spots across the 15 players. I prefer to take an optimistic view and from what I've seen around colts, schools and juniors over the past 10-15 years, we have players who have the talent to play 5/8 at the top level. What I think has happened is that they have been overcoached or poorly coached to the point of being confused with what they need to be doing. Add to this constant movement of players from 10 to 12 to 15 and it makes it worse.
A crucial part of backline play is knowing where your team mate will be and what he will do in most situations. This is done by playing people in the same position with the same combination as often as possible, not by constant swapping of roles. So if your 5/8 gets injured, you don't move your fullback or centre there, you bring your next best 5/8 up. It's a position in which you need to be playing in all the time as you have to run the game and make decisions quickly or even unconciously. This, I believe is part of the problem which we are facing at super, test and JWC levels, players hesitate for a moment because last week they were playing fullback and the opportunity is lost and they look like they can't play, but it's their preparation and coaching which has let them down.