How long have you been on Rogaine? The longer you've used it (whether or not it has regrown anything or is just maintaining what you have), the more drastic your loss will be after stopping it. Ex: 2 month or less - probably only a slight loss; 6 months - moderate loss; year or more - heavy loss.
What I can say is that, at this point there is simply no way you can be sure that it is just the Finpecia that is working. What is the most likely cause of whatever you are noticing is the COMBINATION of finpecia and rogaine. And I hate to break it to you, but most of it is probably attributable to the rogaine.
If you have been on rogaine for quite a while and stop, you are taking a big risk. There is absolutely no guarantee or evidence at all that finasteride, by itself, will regrow the same hair lost from discontinuing rogaine. And even if it did regrow some hair in those areas, it most likely wouldn't be as much as rogaine did; and I can almost guarantee you that it wouldn't regrow the amount that a combination of the two would.
These answers proabably sound like they just keep getting worse for you. Trust me though, I went through the same hatred of the thought being stuck on Rogaine "forever" (like so many others here have), but you will find that when your hair loss gets noticible enough, you will do whatever it takes to regrow/maintain the most amount of hair possible.
Best of luck in whatever you choose.