>See: the clauses saying "must reproduce/retain the above copyright".

Yeah, you will see the copyright if you are working at the company. Outsiders would never know it's a fork.

Read the licenses more closely.

If you're fine with violating the license, then there is nothing stopping you from closing the source to GPL licensed software without a word of credit. I'm not sure what your point is.

No need to violate the license, just close the source and no outsider would know it's a fork.

See clause 2 of both BSD licenses.