Specifically, its effects are permanent. The only reason the ghosts can come back so quickly is because Prime Evil has built a high-speed tunnel (the Scareway) between the dimensions. When the Ghostbusters' fathers did their thing, this tunnel did not exist, and ghosts had to wait for a hole to open up, resulting in a bottleneck effect. This tunnel can also open anywhere and anywhen, which is why the heroes need to time-travel.