Some complained that Ragna 'suddenly become strong' in CF as he kicks everyone's ass there but for fans he was mostly known as 'underdog' before CF. Is it just because of him being talked down a lot despite his feared reputation or his 'many losses'? This is his battle breakdown, story mode canon ones:
NOTE: 'Base' Ragna = Ragna without active Grimoire (usual non Unlimited Ragna, not the crippled one)
Before CP True Ending (more details on Quote below)
TLDR/Conclusion for pre-CP True Ending:
All of his struggles are against an enemy that has equally or even more fearsome reputation such as (mentioning from CT to CP ones):
Rachel (Daughter of famed Clavis and dubbed the Onlooker of the world)
Hakumen (Leader of legendary Six Heroes who slayed Black Beast)
Hazama (Which houses Terumi)
Kagura (Head of Mutsuki, most powerful Duodecim in NOL, also ranked General)
Azrael (Have many, nicknames that each one sounds as dangerous as "Grim Reaper")
Then, most of his struggles/loses are also when Ragna is handicapped
And in conclusion for Ragna, base Ragna is actually stronger than you think. The only ones matching him/stronger back in CT was Rachel, 20% Hakumen (Ragna's stronger with Grimoire active), and Nu. And since CP he's not below 20+% Hakumen, contrary to popular belief. He's already matching him without active Grimoire and the other losses of his was only because Celica crippling him.
Post-CP True Ending and CF (more details on Quote below)
TLDR/Conclusion for post-CP True Ending and CF:
CF Ragna isn't Ragna suddenly becoming powerful for no reason. Ragna is just being the same Ragna in CP True Ending before (In which Ragna improved himself with skill).
Also, Ragna isn't winning because of Doomsday effect, because Ragna is already stronger than most Qualifieds (such as Litchi, Makoto, Bullet, etc.) since even first game and Ragna without active Grimoire have become on par with the Qualified powerhouses like Kagura or 20+% Hakumen since CP True Ending.
You did say above that "some returning characters won't be playable like Jin because he can't fight anymore."
But I'm just pointing out there that we lost more CF casts than you think. I mean that yeah BB5 can happen, but how about the loyal fans of the series who only play for fighthing part (which I think 80% of BB playerbase)? They'd probably be mad for the loss of the casts, due to "BS story reasons".
Mori originally wanted Nu and Lambda to stay dead, but due to fighting game demands (complete playable cast), Mori revives them and brought them back and Mori doesn't want characters to be playable for the sake of being playable.