You can explain bonsai all you want. It's a lost cause. They love the plant too. It belong to them. Move on.
By the way, telling someone that you're gonna chop the plant down to a foot, push it into a small container and trim its root regularly will not sway them to give it to you.
Seriously, it will, most likely, make them think you are some kind of sadistic plant freak that they'd rather not give their much-loved bougie to.