We most certainly will not be forgotten, unless your history is purged by some other in game events.
The 10 year story is 10 real life years, which equates to 520 Elyrian years. As long as people keep up with writing down the history of the world, then within the first year we will be making history.
As far as your questions regarding paying to play again:
1. We pay about yearly anyway with sparks of life which last us about 52 weeks each, so worst case we would continue to pay yearly to play.
2. If SbS was inclined to make a sequel to the game instead of adding another 10 year story, it could be feasible that at the end of production of a sequel we pay to play said sequel. Much the same as we pay for other games when they come out with sequels.
I do not think we will be forgotten though. The way SbS works we have already been influencing the world with events like the Searing Plague, and we haven't even gotten to test phases yet. If they make a sequel to CoE then I almost guarantee the events of CoE will be reflected within the sequel.