Yep, it probably would. Let me be more specific as to why I think that it would do poorly in theaters, Or at least, if it were released today.
Yes, Creed has sold millions of albums and was/is extremely popular. Yes, a movie could be done about the "whole" career of Creed, since technically, they've broken up. But Creed is not old news yet. Look at the successful movies about Ray Charles and Johnny Cash. I have little doubt that a movie of that quality could be made about Creed. But a big difference is that those movies are about artists who were at the peak of their popularity in the sixties and seventies. Creed was at the height of its popularity four to five years ago. They still get airplay on rock, and occasionally pop, radio. Though they've been disbanded for three years, they are not really "old" news. Stapp is still touring and performing the Creed hits along with his solo material. Despite its massive popularity and success, Creed has not achieved the "legendary" status like Ray Charles and Johnny Cash have, something which helped propel their movies' success. If a movie was made today about Creed, people wouldn't be saying "OMG, I loved Creed, takes me back to my youth. I can't wait for this movie, it will bring back so many memories." No, many people still remember all of the slander that Creed got, many people still irrationally hate the band, and we would hear nothing but bitching from people who were angry that Creed got a movie, but that a more "deserving" band didn't. Many people still hate Stapp, still think that Creed was a Pearl Jam rip-off, still are annoyed about the whole Christian controversy, and still remember being tired of hearing My Sacrifice and With Arms Wide Open on the radio all the time. To green light a major motion picture about Creed nowadays would unleash a shit storm that would skew people's view of the movie, and no matter how well-done it was, it probably not be as successful because of this. Creed just hasn't reached the status where people can look back on it with pure nostalgia, and the hate for the band still exists and would hurt the movie.
Also, to do a movie about Creed right now could be very premature. As I said before, Stapp's still performing the old Creed songs, and is putting out solo albums. The other members are alive and well in Alter Bridge. These guys' stories aren't finished yet. What if Creed reunites and sells a couple million more albums and is even further solidified in rock history? What if Stapp does some more outrageous things that could play out great on the big screen? If a big movie about Creed were made now, it would not include any of the possible future history of the band, and would ultimately be looked back on as an incomplete telling of the Creed story.
If you ask me, wait a decade or two before doing a Creed movie, when there could be a more full image of Creed's career, as well as those of the individual band members. Hell, wait till the band has passed away. That's what they did for Ray and Walk The Line. It sounds kind of morbid, but it worked, and then the directors wouldn't have to worry about not covering the complete story of Creed. By then, most of the hate should have burned down. Sure, musical cholars and critics will probably sitll be debating the merits of the band. But in the general public, all that will be left will probably just be memories and nostalgia.
So that's when I think that a Creed movie should be made; when the band members are old or gone, and Creed is just a part of music history, not a living, changing entity. If they waited that long and made a stupendous movie, it could be awesome and very successful. But I don't think that it should be made nowadays. Creed is still evolving, and its detractors are still out there.