I can give my opinion on it. Saying that it's silly is an opinion; although, I think I still have a pretty good case for why it was a good decision for Apple to go back to what they had been doing for YEARS.
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Apple had always had a normal background image associated with the login screen. Then, in Yosemite (or whenever), they decided, "Nope, let's make it look as though you lost your prescription glasses before sitting in front of the computer."
They're going back to their [great] roots by bringing back this non-blurry design element to Mojave.