I like the spirit of the change, as it moves towards making it easier to append ourselves as authors of files with existing copyrights rather than appending manually.

However, the implementation has some flaws (see comments).

For this input:

# Copyright (c) 2014-2016 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.

I would expect this output with insert:

# Copyright (c) 2014-2016 The Bitcoin Core developers
# Copyright (c) 2020 The Bitcoin developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.

And with this new output as input, I would expect insert to make no changes.

What is curious to me is that insert and update are separate commands. On one hand, this makes sense: if you author a new file, you insert the entire copyright. If you modify a file that you did not author, you update the (hopefully) existing copyright.
However, with the substantial linting, modification, and automation we've been adding to the codebase, I see no issue in always adding ourselves as authors of every file automatically. I think there's an argument to merge the behavior of the two so that
as much of the copyright that is missing gets added as needed, or only an update to the existing line is made.

There's a strong hint here that we should add this as a linter so that new files get copyright added automatically and newly backported files get a new line appended to the copyright automatically, and every file touched at the beginning of the new year will get
bumped automatically. In essence, we won't have to waste brain cycles on this problem ever again if implemented with care.