Just because a skin was made doesn't mean that it take the place of another skin. Riot have stated many time before there are always a skin in development just because something was release doesn't mean another skin is not gonna happen.
For example the valentine day skin earlier this year when it was release people were upset that Sona got another skin when champ like kennen haven't gotten a skin in 4 year, well guess what happen the next patch bloodmoon kennen. Just because something there is never really a case of a skin take up the place of another skin.
Another thing is that Riot have release schedules. Bring up the Valentine day skin again people were upset that Kennen didn't get a skin but complaining on that Valentine patch about isn't going to help because it the Valentine day patch Riot will release a heat theme skin now unless you wanted a heart theme skin for kennen he would never get a skin on that patch.
Now with the recent skin they are release as a group unless you was expecting El Pollo Swain he wouldn't get a skin this patch. Now I'm not saying it is impossible for Riot to release something else along side the El skin because we seen them done stuff like that before where there is a batch a theme skin coming out with another separate skin but that could also come down to there schedule and whether or not it finish or not.