To be fair in this case it is the same people doing the rewrite as the original version so I guess the theory is that the hard-won knowledge has not been lost and can be integrated into the new code.
The second-system effect refers to the tendency of small, elegant, and successful systems to have elephantine, feature-laden monstrosities as their successors. The term was first used by Fred Brooks in his classic The Mythical Man-Month.[1] It described the jump from a set of simple operating systems on the IBM 700/7000 series to OS/360 on the 360 series.
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FWIW I hate the posting of a link without describing the content in some way.