Destiny lovers 2 did it worse. Theoretically a good cutoff point for a GraPara2! announcement would be when Sakura learns the truth and it just leaves off with one final panel showing her moment of truth face and that would honestly make people probably pick up GraPara2!
However the chances of a GraPara2! is a whopping 0% if you ask me, and I'm one of the people that actually wouldn't mind something like it. 3 years is a long time, it's been a good run even with the missteps towards the end, let's just see the ending and then move on with a "lighter heart" instead of Airi's "warm heart" approach.