How do I wrap up a campaign when players aren’t keen on risking their characters?

First, ask whether they still want to reach the campaign’s ending, just not at the cost of their character. Those are different things. You could agree that this character’s death means the player steps out, or decide together that the final stretch won’t require a replacement character. That lets you plan around what they actually want instead of assuming everyone shares the same idea of a satisfying ending.

Ask what the other player means by being underpowered. Are they worried about the fights being too hard, or that their characters won’t feel ready for the story’s climax? Once you know which it is, you can adjust the finale to suit the group, or give them a chance to prepare. A final challenge doesn’t have to be deadly to feel like a proper ending.

You can tell the group that finishing the campaign matters to you, while still leaving them room to say what kind of ending they’d enjoy. Maybe the last sessions focus on resolving the main story without making character death the price of continuing. If some players don’t want to play through the ending, you could also ask whether they’d prefer a brief wrap-up rather than stretching out sessions they’re no longer enjoying.