It's the same situation as when he told momo to bite Kaido he wants to show her she doesn't have to be afraid, that the trauma caused by Saturn shouldn't control the rest of her life
Well in Momo's case he is the future Shogun
He will have responsbility of Wano
Bonney is just 12 years old, daughter of Kuma
Actually both of them can just go back to Revolutionary, and the army will protect Bonney
Like Rebecca, she can just never fight again for the rest of her life
Old Luffy finished the job for likes of Chopper, Robin, Nami, Usopp, Vivi etc
Same Chopper and Robin that can clearly fight and older than Bonney