Practically, I sometimes worry about this regarding my own feelings towards weight gain as a feedee. I worry that once I've gained as much as I'm comfortable with that I wont know what to do. I don't want to return to miserable dieting again and I don't want to get into a cycle of yo-yo dieting just so that I have the freedom to gain again. (Though that's only because I hate the thought of losing weight and dieting because I do relate them to unhappiness.) This is why I would never intentionally gain weight though, I want to enjoy this freedom for as long as possible.
However, in terms of a relationship I don't find this a worry. I see my weight as a side effect of my happiness and security. As my relationship grows, I see my body grow, and I find it all very romantic. I also think that there is a stage when two people become so close and are able to share a life together, for example in the sense of marriage. The development stabilises, and in the same way, I expect so will my weight, because I would have acheived the ultimate level of security.
For me I suppose it may be different as I'm young. I've always thought that I'll be a fat wife and mother one day, so to me weight gain is just a natural part of growing up and becoming the fat woman that I want to be, and I find that very sexual. I don't have a weight goal; I just have a life goal. When I think of the future, of having a home and a family, being a fat woman is all part of that, and I find that romantic in its own way, different to the sexual side of weight gain.
I don't think the fetish will go away, as I'll always find being fat/getting fatter sexy. However, I think that once I'm as big as I'm going to get, there will be other advantages which appeal to the fetish that being smaller and gaining weight don't have. It'll be different, but not necessary less

I think it also depends on how much of an FA the feeder is too, so that these pros and cons also apply to them.