On almost every alteration, someone suggests knocking it all down and starting again. We nearly always argue against it.
It is usually cheaper
Foundations and walls that are already standing represent a large part of the building cost. Keeping them leaves more of the budget for the parts you will actually notice.
It is greener
Making new bricks and cement uses a great deal of energy. A wall that stays in place has already paid that cost.
It gives the house a history
An old wall with a new opening in it tells you something about the place. We like houses that show their past.
Of course, some walls have to go. We just make sure that is a decision, not a default.