The only reason we tend to get tries is when defense is tired. If it was fresh I dont think it matters how good your attack is.Less points.
If only could sub 1 player and that being the halfback it takes away his skill of planning under fatigue. This is a skill in itself.
My opinion (based on my observations) is that fatigue is not a factor in the game anymore and I don't think it is coming back. Defenses are just too organised now and sports science training has everyone so fit and well prepared.
If fatigue ever was the source of opportunity for attack I am of the opinion we need to look at new things like reducing the number of players on the field or otherwise trying to break up that dominant straight line of defense.
I also think the main thing that has led to attack drying up is that attacking teams don't take risks and ask questions anymore. It is just much easier to take the 7 or 8 easy metres and kick for the corner.
Attacking now is about trying to get a fast play the ball and getting penalties.
The solution in my opinion is not to try to bring fatigue back but to try to get it so that the game is ahead of the coaches in terms of what you have to do in defense. The coaches are just so on top of the game in that regard. The ten metre rule has had its day.