Regrowth is just a copy. It can only ever be old regrowth. Karri trees do not live for thousands of years. Scientific studies indicate karri trees on average do not live much more than 250 years. I am not aware of anyone claiming that karri trees live for thousands of years.
Individual trees can live for a thousand years - if given the chance.
They don't just die and fall over at any particular age. To chop them down on the pretext that one day they will die and fall over is as crazy as saying we should kill people off at 50 or 70 years of age because they will die one day. Furthermore, standing dead old trees provide an important habitat. The community is constantly told by extreme environmentalists that good-quality logs capable of producing sawn timber are being burnt as firewood or converted to woodchips. No one can guarantee that no sawlog ever has been burnt as firewood or turned into woodchips but the percentage would be very small.