I mean, it -was- kinda necessary because of two reasons - both of them revolved around storytelling:

Jyn and Cassian were able to sneak inside of an Imperial Base. They went undercover and woddled inside of the base for as long as they wanted basically. That's fine for the movie, but in actuality that'd make them too resourceful, and prior to their return to the Rebel Base, they'd be like, considered super spies and be basically at the front of the whole Rebellion. It makes no sense for them NOT to show up in ANH and so on, because if they'd have survive, they'd have become legends and terrorists to the Empire, top wanted.

Also - Rogue One had to kill the cast off to show how serious and dangerous the Empire really is. How dangerous -Vader- is. I mean, yes, seeing Vader slice apart an entire hallway full of rebels is fun and cool and badass, but from a storytelling perspective it also makes sense and is important, because it puts into perspective how much of a big deal Vader's role is in the next movies. And the Empire. And the Emperor. Without any of this, Rogue One would have failed to depict a very much needed detail and would've failed at another important detail, which is world building. The ending to Rogue One makes sure you -realize- as an audience, that you absolutely do not mess with the Empire and make it through to the other side. You do not mess with Darth Vader and live. Luke and his friends were an exception, because of Luke and Obi/Yoda. Jyn and her people didn't have this advantage, so rip