Tim 'avatar' Bartel wrote:
of cases where people (in Germany) notice an '...or later' clause and sign a contract anyhow because they know, that this clause is ineffective.
German law applies *in Germany*, not *to Germans*. As soon as they (or their works) move beyond borders, they are exposed to other legal systems. (Go to China and kill someone -- you might be sentenced to death.) So if the clause is ineffective under German law, you can feel "safe" only as long as you (and your work) stay in that country.
Suppose the WMF in the year 2057 decides to use your Wikipedia articles in accordance with GFDL version 17. You're in Germany and claim that you never legally agreed to this, and you sue WMF for copyright infringement in a German court of law. That might stop WMF from reusing your articles in this way in Germany, but it doesn't stop WMF from reusing your articles in this way in Mexico. If you're going to sue anybody for copyright infringement in Mexico, you must find arguments that work under Mexican law.
The state of Bavaria claims they own Hitler's copyright, which they confiscated in 1948, and sued a Swedish publisher of a translated "Mein Kampf" (1992). But the Swedish supreme court in 1998 said a state cannot legally confiscate copyright for the purpose of blocking publishing, since that would mean censorship, and turned the case down. The book is sold in stores (part 1, ISBN 978-91-7123-100-0 and part 2, ISBN 978-91-7123-101-7).