Hi!
For one reason...There are already tons of public LIDAR datasets with point data down to millimeter. I've even seen monuments and public art sculptures that have LIDAR datasets down to millimeter precision using WGS84 base references.
OK, but: 1. For objects we store in wikidata, what exactly is known with this precision? E.g., if you have https://www.wikidata.org/wiki/Q5577191 - which part of it the coordinate specifies up to millimeter? That is a big monument, lots of possibilities. No way to know. 2. For objects we store in wikidata, this is probably not stable - even if we somehow defined that the point we know up to millimeter is the tip of Schiller's nose in Q5577191 - that probably would change with weather conditions, erosion, ground shifts, continental drift (on millimeter scale, that becomes relevant), etc. 3. For any practical use - such as querying - having data for one statue only with millimeter accuracy is useless, to really do some calculation where it matters all statues (or at least all statues, say, in New York - or at least most of them) should have such accuracy. That obviously won't be the case.
For this reasons, I don't think millimeter accuracy would ever be relevant for Wikidata.
Having it all queryable in Wikidata ? hmmm... not for me, other data catalogs and GIS systems handle that job.
Exactly.