I understand why Glock did this for .40 S&W.

Why did they do it on the G19?   Wasn't it already "perfect"?

Hes anyone replaced the dual spring/rod combo with one for Gen. 1/2/3 G19?

Ease of manufacturing.  The Gen 4 frame dustcover is the same on both 9mm and .40 guns.  The Gen 4 slide recoil guide hole is needed to match the Gen 4 frame dustcover.  An identical recoil spring set-up is used to keep manufacturing streamlined.  Why would you machine the slide recoil holes to take a different recoil rod assembly on the 9mm guns?  If anything, install a lighter spring of the same physical design (they did) into the 9mm guns.

This is the same reason the Gen 3 guns in 9mm got the extra locking block pin that they never needed.  Why have a .40/.357 production line seperate from the 9mm production line when the only difference is a $2 pin?

Part commonality.

The previous Gen 3 rods will not fit in a Gen 4 frame.  I believe someone is selling an aftermarket single spring rod assembly though....

IIRC the replacement interval on the Gen 3 recoil spring was 5000 rounds.  The new design is supposed to extend that a fair amount.  

The new setup also has noticeably less recoil.

If the new design proves to be as reliable in practice as the Gen 3 pistols it will be a very nice upgrade.

I have a Gen4 17.  About 1000 rounds shot and only one failure to eject.  That was from the first loaded mag using handloads.  From that point on, everything I have tried has been just as one would expect from a Glock.  Recoil is mild.  Not too crazy about the accuracy when compaired to my P99 though.  I may need to practice more.