If enough sites started to do this, google would stop respecting it. This isn't blocking google from indexing, it's asking google not to index..
Normal users will start to use another newspaper
Wikipedia should do this as Apple and Google are showing Wikipedia results as their own, robbing, IMO, Wikipedia of importance. Wikipedia is large enough that it should have their own search engine, likely with more relevant results.
Identical situation when Facebook was asked to not show previews of the news articles, because of the ePrivacy directive. Could this go for the same legistlation?
It does not help Wikipedia to do that. The content on Wikipedia is licensed so that Apple and Google can show the content from Wikipedia (and this is by design, not a loophole). If the users can get access to the encyclopedic content more conveniently, that is still in line with the project's goals, even if that content reaches the user indirectly via a third party.