That explains all those harsh, brittle guitar tones on albums from the 60s, 70s, and 80s.
Dude, seriously, it doesn't take anywhere near 30 years to break in a speaker. This is madness, and it is a quality control issue. Speaker break-in should only take about 12 hours, and the sonic difference shouldn't be this dramatic before and after.
Small choices in construction can make a big difference. For example, with Shure microphones - take a vintage SM57 (with minimal epoxy holding its components in) vs. a modern SM57 (with the components bathed in epoxy) and check out the sonic difference. In this case, I personally feel both can sound great, but they do sound substantially different. An SM57 and SM58 are the same mic, but the enclosure alone makes a big difference in the sound.
Point is - small choices can make a big difference in sound. Even if they have all the old Celestion factory machines and components in China, there's still a lot of aspects of the construction of the speakers that could have changed. Maybe it's something as simple as glue choice or technique. Maybe these choices make the speakers more "road worthy." I wouldn't doubt that any of these things are possible, but it's purely speculation - in any case, it's clear to me that
something is different, and has resulted in
substantially inferior sounds, even after the new speakers are broken in.