The Battle of Scarif may have been the Rebellion’s first victory, but they lost a lot.
Though a win is a win, Star Wars has long been a war of attrition. The Rebellion has less to lose and less to fight with. So every set back is felt significantly.
The Battle of Scarif put in some big set backs to the Rebellion.
The Rebel fleet made a big impression on the Imperial Starfleet at Scarif. But during the battle, they lost many of their fighter squadron, including all of Blue Squadron. Many of Gold and Red squadrons were lost (including Red5, though they would find a replacement a couple weeks later).
Admiral Raddus’ quick thinking led to a Hammerhead Corvette – – fatally driving itself into a disabled star destroyer and taking out the global shield generator (though we’re assured the crew did make it off).
And lastly, the Profundity, Admiral Raddus’ ship, was lost to Vader and the Imperials. It’s not clear what happened to the person, but his ship was certainly lost.
Strong leadership is the cornerstone of the Rebellion. Admiral Raddus seemed like the kind of take-charge commander that inspired greatness from his followers. The entire victory at Scarif can be attributed to his and his crew’s bravery and cunning.
Though Episode IV, A New Hope shows us the prowess of the (remaining) squadron of Rebel fighters, we don’t get a sense of the type of military troops the Rebellion has until EpV. The Rebellion lost a LOT of their ‘best’ troops at Scarif. Melshi, Pao, Bistan, Tranc, Sefla (read the Rogue One novelization to get their names), and many others were terrible losses.
And lastly, the Rebellion lost Rogue One itself. This crew of misfits, brought together through extraordinary circumstances pulled off an impossible mission. I’m sure the Rebels, without the thought of having the help of the Jedi, would have solely depended on the bravery of people like Jyn, Cassian, and Bodhi. And without Baze and Chirrut, the true last remaining legacy of the Jedi – not Jedha – were destroyed. Until Kenobi that is.