I normally don't (and probably should not) get into this little squabbles, but I am getting a bit weary always reading through me... so let me see if I can help bring this all together in the spirit of goodwill...

I think that instead of using the term 'relevant' that Wikidata should instead begin to adopt a policy of 'useful'.  Just a slight tweak to its policy with this one word change and it opens borders and collaboration and less debates like in this thread.

My thoughts....

Wikimania's site was useful to me.  Yes.
The information it has could be added or copied to Wikidata.  Yes.
Or the information could stay where it is... on Wikimania's site.  Yes or No.

So the question for all to decide is one of...

"is it useful to copy some of Wikimania's information data into Wikidata"
"will doing so make the data more useful to everyone"

Questions like those should hold the highest court above all others, and I think that is where Gerard is trying to help ask.