Well, it was originally meant to be a multi-venue game like the book is. As for Promethean. It's made for the ST to walk with the PC throughout the entire game. As you get set goals and such to complete.
Not to sound linear, as the majority of your adjectives are in fact hidden from you.
The summary of P:TC is that you're a monster, you was born a monster and ocstrasized from humanity and nature itself.
Your major goal is to obtain Mortality and become mortal. To be accepted. Key themes are alienation and grotesque circumstances.
Now this game is designed to make the final goal obtainable [as it has actual rules for becoming Mortal] so it doesn't make it feel pointless.
And yes, you do not have to stop playing if your character obtained mortality. In fact, they could either do three things. Stay a Mortal, Awaken as a Mage or be Embraced as a Kindred. When worse comes to worse, they die as a mortal and another Promethean brings them back as a Promethean [woowee, way to start from the beginning all over again.]