Question pretty much sums it all.
Should all questions relating to wordpress be redirected on wordpress.stackexchange.com and render the wordpress tag useless?
No, I do not think so. You should look at the FAQ for Wordpress to see what is on-topic.
Coding which has to deal specifically with the language and not necessarily with WordPress is actually on-topic for SO.
Plugins, theming, basically anything directly connected to WordPress engine will be on-topic there.
Want to fix your Theme options? WordPress
Working on your PHP function (A Wordpress function), but the solution is independent of what CMS? StackOverflow
Moving 10,000 questions just isn't right either, consider the amount of effort mods on both sides would have to undertake to clean up migrations. Better to just leave questions (for example from 2009) where they are currently located. The scope change should only affect new and closed questions.