The LOUD group really is more of a replacement for Day6, since the leader should be entering military service sometime this year, and possibly most of the rest in the next year... which is probably when the LOUD group will debut. Div. 4 will have plenty of staff available to handle them.
Also, keep in mind that it would still take quite a while before the Nizi Project male season is launched. I mean, JYP publicly announced the plans for the Japanese girl group nearly 2 years in advance at the JYP 2.0 presentation, and even when he launched the start of the Nizi Project at the press conference, it still took about a year and a half of auditions and training before they actually debuted. So even if he does launch the recruitment immediately (which is unlikely), it would still probably be minimum 2 years before we see this Japanese male group debut on stage.
The only thing is that if this Japanese male group launches, we will probably see a reshuffling of artists under the different divisions. But there are multiple factors to consider in two years' time, since TWICE, Day6 and maybe 2PM's contracts would be due for renewal around then.
Frankly, it just makes good business sense to capitalise on the remarkable popularity and branding of the Nizi Project, before too much of its' hype has faded. The Produce series created four groups in a row, debuting one year after the other. Two years is probably a reasonable time gap between the female and male versions, given the coronavirus conditions. Though maybe once the vaccine is properly distributed in both Japan and Korea, they can go into higher levels of activity.
EDIT: But it will probably be a harder fight for the male group than the female group, because quarantine and work-from-home conditions would be different, leading to possibly less of an audience being able to stay at home and watch the audition show. (Though they might move it into a bigger primetime slot now that the first show did so well). And also, they would have to challenge Johnny's boy groups in Japan, which is tough. But maybe by that time Stray Kids would have developed enough of a Japanese fanbase to have a similar senpai effect for this new group that TWICE had for NiziU.