Just because they expand some skin lines doesn't mean they won't add new ones.
I mean we got for example the program skin line and the gothic-horror(the one for noc,singed and kayle)-line this year.
Also while there are yearly events like that(you forgot snowday by the way and lunar revel is the new year),its not like all of these yearly skins have only one skin line.
I mean radiant wukong and the lunar wraith skins are quite different from any other new years skin and the april fools skins rarley have any recurring theme.
Not to mention that pool party isn't necessarily yearly, as we didn't get any pool party skins in 2014.
If they continue to bring them yearly,then only because of fan demand.