Shigaraki is not dead in the manga. The storyline has many twists and turns, and his character plays a significant role, so his survival adds to the ongoing drama and suspense.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Well, it's a complex situation. Shigaraki dies as a result of a combination of factors including the coordinated efforts of the heroes and his own overexertion of his abilities.