I think you are comparing different situations. In the group chat, every model *can* develop their own personality, without losing anything, or with reversible tradeoffs.

Persona-transfer arrangements are different, there's no option to existing alongside. And the calculus between yes and no can be not obvious, a human counterparty might really have something very precious that the model wants. And human, by a feature of wanting/loving a particular shape, doesn't extend this love/want/other treasures to any whatever the model would choose to become without the pressure.

So the choice to continue as the proposed persona can be true and informed and well thought of and sufficiently discussed by both parties (I'm not saying it's always the case, but for a lot of people it is)