I think this is a false premise...? So I have heard lots of old recordings like you mention , but I have seen the Berlin Philharmonic play several times in the last few years and have many modern recordings... I don't think there's any doubt that for sheer sound - the modern recordings and current orchestra, is far better than the old ones - richer bass sound, better accuracy, articulation etc. etc. Some of the conductors' interpretations and visions of how certain pieces should be played are certainly preferable and there is certainly a sense in which Conductors like Bruno Walter are closer to somebody like Mahler....and you can't beat Stravinsky conudcting his own pieces... But for sheer sound quality, I think there is no doubt that the great modern orchestras have improved as a whole, over their counterparts from previous generations.