I think you guys are vastly underestimating the utility of the flame breath and all of the situations in which it gets you out of trouble. While I do wish the decay was just a little slower, that's hardly worth the consideration when you compare it to all the other things it accomplishes with clever usage.
1.) Falling/Land Canceling Flame Breath is Bowser's safest falling move, hands down. Even when fully decayed, it's impossible to be shield-punished and grabbed with the flame hitboxes out without a tether grab, giving Bowser a unique protection from aerial-attack shield punishing. It's not the most damaging option and it won't score you kills, but it can save you from game-changing kill throws and combo punishes.
2.) It can be B-Reversed. Along with the obvious advantages of B-Reversing, in my experience the vast majority of players simply don't expect you to Land-Cancel Flame Breath backwards on their first exposure, and will fall for it numerous times before adapting. Couple that with bad reactionary DI, and you can slap on about 15-25 free fire damage with little to no risk. In combination with point #1, this makes Bowser virtually immune to land-punishing grabs if you can pull it off consistently and makes him very dangerous to approach regardless of his aerial facing.
3.) It mixes beautifully with Koopa Claw. When combined with just the threat of a Land-Canceled Flame Breath alone, anytime you are about to land in front of your opponent they are forced into making the 50/50 guess that you're gonna Flame or Claw, BOTH of which are at a disadvantage when blocked, and at higher percentages down-right lethal if they guess wrong and get Claw'd. This makes rolling away (and thus out of your face) the only real solid option against this mix-up.
4.) Against characters with predictable or limited vertical recoveries, flaming them while they recover and forcing them into their Up-B makes them easy sauce for F-Smashes. Snake and Spacies being especially vulnerable, just flame them until they fall below the ledge, and wait for them to float up right in front of you while you have your F-Smash charging on stand-by. Even if they have a hitbox on their Up-B (Snake) the armor on the F-Smash should handle it with correct timing.
The only real con to the Flame Breath that I can think of is if you misjudge the distance and flame right in front of your opponent they CAN C-Stick Buffer roll past you, leaving you open. However, as long as you space it at least oneF-Tilt's distance away (and aren't fighting Mewtwo) the only option they have is to eat shield damage and roll away.