@Eri-Natori Because unlike the NA version which has started only recently, JP players may be atached to 8 years of progress that they're not ready to leave right away. We may also want to still play existing content from the base PSO2 even after NGS launches, since it's quite possible that NGS won't have alot of content at launch.
While PSO2 NA started with quite a sizable amount of content (even in the betas), the same wasn't true for JP. PSO2 JP started with a total of 4 areas/expedition, 1 urgent quest if I'm not mistaken, 1 time attack quest, 4 story chapters (not the full episode 1 like the NA beta), 3 classes, lvl 40 cap and only normal and hard mode difficulties. It took 5 months until we got our first major boss urgent quest (Dark Falz Elder).
It's quite possible that something like this will happen with NGS, not having much content at all at the start, and naturally content updates will be much slower than NA currently, so the base PSO2 will provide a nice distraction while NGS starts getting more content. Once NGS has a sizable enough amount of content, it's possible that the base PSO2 will end for good.