Boiling it down to 'Singer is holding back the franchise' kinda disregards everything else that made Deadpool such a hit. It was not just about being comic book accurate (most people don't care about that anyway), it was the combination of the marketing, Ryan Reynolds, it coming off the well-received DOFP, the fact that the character himself is a comedic character (funny characters are better received by audiences) and the oversaturation of superhero movies in this current time period which would make Deadpool seem like a refreshing breath of fresh air in comparison.
It's literally a lightning in the bottle situation that you really can't say that all movies, as long as they're comic book accurate, would be billion dollar hits.