The patina layer is a protective against corrosion.
“The patina layer is a protective against corrosion.”
I wish our republic had a similar protection against democracy.
A metallurgist I dealt with ages ago explained the concept to me. Most of the various metals subject to corrosion and/or specifically oxidation have what is termed a sacrificial component. Some, like aluminum have it integral to the metal itself -- aluminum oxidizes densely enough to form its own protective coating.
As you mention, copper forms a patina. Some, like core-ten steel, are designed to rust so densely that the rust protects it. Poor zinc coated steel can develop rust in the long term bad enough that the protection fails. Other zinc on barbed wire may last a hundred years or more before rusting away. Some metals like gold do not oxidize enough (or at all) to fail.
The patina layer protects against further corrosion. It is in itself corrosion so if it protected against all corrosion, there'd be no patina.
Plus removing it will mean removing copper from the statue. And if they want it to remain copper-colored and not be just as green in 20 years time, they'll have to apply a new transparent protective coating, which itself will need regular maintenance and repair.