Summoners War is out for something like 1.5 years now, i've been playing it for 8 months... it's not really slowing down - so that's a good news for this game, however, the major thing that keeps people playing is true competition. Well designed arena and guild wars and probably "edited for a censored word - a "low level character"" arena coming up soon.
Arena here is kind of a joke, an afterthought if you will.... thankfully devs have hinted an overhaul.
No chat and guilds are major detriments as well. And let's not forget the money making model. Summoners, employ a model that does not guarantee you anything really. No "special $99.95" packs that out of the bat give you huge advantage over the majority of playerbase. all these affect longevity.
If i have to grade longevity of this game at the moment it's prolly C+