I had no idea! lol
It’s not assuming anything, it only explains how features tagged with brand:wikidata can show up in the results. I know that it’s close to impossible for all features ever to be correctly tagged at all times, unfortunately.
But that’s a different issue (as I already said) than the “correct” way to tag a particular feature.
If you agree that, ideally, KFCs should be tagged with brand:wikidata=Q524757, and also believe that it’s close to impossible to have all KFCs correctly tagged at all times (see above), then what’s the point of discussing the “correct” tags for KFCs with no brand:wikidata=Q524757? We’re back to square one: even if we agreed on a common tagging scheme for KFCs with no brand:wikidata, how would we make sure all KFCs with no brand:wikidata will follow this scheme, given that we can’t make sure all KFCs have brand:wikidata?
That’s fair enough, but again not a tagging issue because if KFC is called “ケンタッキーフライドチキン” in Japan, it should be tagged that way.
A hypothetical search system could return more relevant results (e.g. closer to you) if it knows where you are (or where you want to search).
I don’t know particulars, but in case you mean the search feature of the main osm.org site, I believe it doesn’t know where you are and doesn’t take into account the bbox currently shown on screen either. And if the site doesn’t know where you are, IMO it makes sense to include results from all around the world.
Again, I don’t know particulars so I can’t comment on which would be easier.