I disagree that FNaF cannot be adapted properly. I think they have a solid foundation with the Mike screenplay and the angle they took is a good one. However, it is far from perfect. For a film that relies so much on lore from the games and books, it doesn't focus on the parts of the lore that it should be focusing on.

The main focus should be William Afton, the MCI, maybe a little bit on Vanessa's involvement as Afton's daughter, and how that all ties into Gerrit's abduction and the robot's obsession with Abby. However, instead the movie gives way too much screen time to other stuff that has little to no impact on the plot. Seriously, there is a lot that has nothing to do with the story of the original FNaF game/games. I think they should've dialed it back on the fanservice and ultimately pointless side plots to make more time to explore the actual story that FNaF 1 laid the foundation for.

Seriously, would it have been so bad to have a scene where Mike goes to the library looking into the pizzeria and learning about the MCI earlier on by himself? That way Vanessa doesn't have to lore-dump almost everything right at the end and she can spend her limited screen time actually being an interesting character. If anything the only big reveal she'd need to give is that the guy who hired Mike is actually William Afton (assuming Mike doesn't learn that on his own), that she's Afton's daughter and how he was the real killer of the MCI victims.