"substantially" is the keyword that should have been emphasised there, and is actually in my post in the above and the wording that was used.
This is because when it comes to copyright law there _is_ such a thing as something being too insubstantial to be considered copyright infringement. "de minimis" falls under fair use.
> You seem to be claiming that modified copies and partial copies are not copies for the purpose of copyright law.
Lets break it down a little bit more clearly:
> Permission to use, copy, modify, and/or distribute this work for any purpose is hereby granted, provided this notice appears in all copies.
+ You are granted the permission to modify.
+ You must provide the notice in all copies.
If a work is substantially modified from the original, such that it constitutes a new work, it would no longer be considered a copy of the original, which is why you are now granted copyright on the new work at all:
> "Unless sufficient of the pre-existing work is contained in the later work so as to constitute the latter an infringement of the former, the latter by definition is not a derivative work." 2 Nimmer on Copyright § 8.09[A], p. 8-138 (2004); see Litchfield v.Spielberg
If you arrive at this point, where you are no longer a derivative because of the substantial divergence from the original work, it may no longer be considered a copy under copyright law.
For a random example from software, the source code for "alpine" the mail client, and "nano" the text editor have an undisputed shared origin, but arguing that they share substantial similarities today would actually be a difficult case to present.
The permission to modify has allowed you to create this new work, but you are no longer required to show the notice, because what you have is no longer a copy.
Whereas the wording in license such as the 3-Clause BSD license:
> 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
Do not allow you to differentiate between a new work and a copy, and close shut that particular escape hatch.