An earlier review of Many Happy Returns states: "The entire premise of this story centers on the idea that pre-Crisis continuity still exists somehow. The point of the Crisis on Infinite Earths was that most of the universes were destroyed, and what remained of the multiverse collapsed into one single universe with a revised history. There were no other universes."
Actually, they do exist--the reintroduction of the multiverse came about when Grant Morrison and Mark Waid did the Kingdom storyline that introduced the concept of hypertime. Waid himself notes: "The entire rationale behind Hypertime was simply to once more throw open the doors at DC, to remind readers that continuity should follow stories, not vice-versa, and that the DCU should be a place where ANYTHING can happen. We're especially proud about the structure of Hypertime--that is to say, if you want to use it, you can, but if you're a creator or editor offended by it, that's fine, too, YOU DON'T HAVE TO USE IT. It's there as a tool, NOT AS A RULE." (quoted from Bruce Bachand's interview with Mark Waid, Jan. '99 *see [...] for the complete interview).
Given all this, the reviewer's argument doesn't hold. when he says, "The idea of one person creating a new world is insane. Kara's success depends on it being one Earth with only one active timeline. Otherwise, Kara would create a 'divergent Earth' and fail."
But Kara's universe *is* a divergent one, one where only she, not Linda, can play the role that was fated for her. In a sense, one person *does* create a new world. The previous issues had emphasized how different the two Supergirls were--it only follows that one cannot replace the other in a parallel universe without there being some substantial differences in the outcome. Hence another divergent Earth--not the DC main universe, nor Kara's universe, but one that came about because of Linda.
All in all, a really good conclusion to the Supergirl series.