Yes, it sounds like your battery is dead. Nothing really wrong with replacing the smok stick if you like the vape. However, as Baditude pointed out I think, internal battery mods do have shelf lives that are unavoidable, and especially with sub0hm vaping it can be hard to keep a battery working a long time (and smok is not known for longevity, at least the mods aren't.)
If you like your tank, well, you could consider a one or two battery adjustable mod, if you are interested (and it sounds like you are stably vaping). If you are looking for longevity period, I might suggest a manufacturer other than smok (I am partial to teslacigs, Aspire even, and Eleaf as well as Innoken when it comes to durability). You can still use your current tank if you like it, but you may find that you enjoy having more variable wattage.
Best of luck with whatever you decide.
Anna