There is such a thing as an optional extra. But somewhere the story can be continued, even more logically ended, even if everyone has already forgotten what the series had in the beginning, and even in the end, as, for example, it was with Dexter (although there the ending quite implies a continuation, where Dexter can return in the same role, in which his sister returned after her death).
Ray Donovan is a series not just about problem solving, but more even about the relationship between fathers and children, husbands and wives, etc. The series is about relationships between people. That's why it's so sensual and heartfelt in many ways.
It's also about the mistakes of the past and how they determine the future. The film was made through the prism of all this.
Whether the film will have a sequel (in any format) is the question. After all, the story can be developed not only as a sequel (in the future), but also as a look into the past.
The film is no different in production than the regular series of the series. It's just one drawn-out series that puts some sort of peculiar spin on it. Surely the outcome won't appeal to everyone, but it's still good to have Ray Donovan back for a moment at least this way...