What do you mean with lately? The last new FF was FFXIII, which came out 5 years ago. The sequels had to sell of course lower because they didn't have the hype and people now knew they wouldn't like them. FFXV is targeted at a different audience.
FF sells on hype and FFXV has currently plenty of it, partly thanks to FFXIII being not for everyone's taste. It's really hard to tell how well FFXV will do but at least the current signs seem to suggest a victory in favor of FF.
The way you said it "I hope so" or "I may leave this forums if FFV bombs below Splatoon" makes me think you are saying that one deservest it while the other don´t. And FF has been shit for years imo.