Sorry, there’s no esoteric preference for this. There were code changes to support the new toolbar, so it’s not just a matter of swapping image files. Also, the old icons were not Retina resolution. Perhaps in time you will become more accustomed to the new icons and find them easier to recognize. (Several testers reported during the beta period that the icons “grew” on them, and they now prefer them where they didn’t originally.)
I agree. Unfortunately, it was not possible to restore the old toolbar state because (in addition to the images being different) the names/types of the buttons also changed.