Well, Deku's arm loss happened when he faced an adversary whose abilities were beyond what he could handle. It was a high-stakes moment that had a huge impact on the story.
Well, Deku does lose his quirk. However, this event leads to new developments and challenges for him. It's a crucial plot twist that adds depth to the story.
Bakugou could use his words. He might tell Deku that he believes in him. For example, he could say 'Deku, you may be a nerd but you're a strong nerd.' It's a backhanded compliment but it shows he has faith in Deku.