I really, really hope that this isn't the mindset of the development team as a whole. If so, my confidence in the viability of Wikidata would take a major hit.
Yes, collecting the information that goes into infoboxes is going to be important, and yes, centralizing that information so that it can be used by all projects is a worthwhile initial goal. It's not the only thing this project is ever going to be used for though. To say that things that aren't currently in infoboxes aren't worth supporting is, quite frankly, a really awful artificial limit the usefulness of the project.
First off, 'what is and is not a field in a Wikipedia infobox' is a metric that changes over time, and often in large or unpredictable ways. Entire infoboxes have been created and depreciated, to say nothing of individual fields in those infoboxes. Someone might come along tomorrow and say "Yes, in fact, we should include the dimensions of historical buildings in their historical units" or "Yes, in fact, we should list both Old Style and New Style dates [1] where applicable". We're then going to be in the position where Wikidata doesn't have the information that people want to include. Had we allowed those things in from the beginning, and properly supported them, we might have had the information ready when it was asked for. If we only add new fields when people request them, those fields won't be ready until long after they're needed.
But more importantly, Wikidata is eventually going to be used for things other than Wikipedia infoboxes. Those uses are going to happen both on Wikipedia and off, and some of those uses are impossible to envision now. We should focus on collecting as much useful data as we can; not just what's in an infobox today, but what might be in an infobox, or the body text of an article, tomorrow.
Please don't sell out Wikidata's future utility for today's convenience.
First off: our target use case is Wikipedia infoboxes. Do you have examples and
numbers about the usage of such ancient units in infoboxes on wikipedia? If they
are not in main stream use there, I don't see why Wikidata would have to support
them.