Presumably it was tested 10 years ago when it was used with 1.12; shouldn't that be enough? For extensions following the release branch model, which get snapshotted biannually together with MediaWiki, being able to see how far those snapshots go back is a nice convenience. Someone using an old MediaWiki version presumably doesn't care about support, or doesn't have a choice.
(I can see how the "supported MediaWiki version" terminology could be misleading for something very old that the maintainer, if there's still one, won't entertain fixes for, but that's my mistake, the infobox doesn't use that wording.)
And I don't think using the field for "oldest MediaWiki version for which there is a compatible extension version" would crowd out more useful information - you are supposed to install the extension from the release branch matching your core version, so knowing what older core versions the extension's development branch or latest stable is compatible with seems quite useless.
For extensions using the master compatibility policy, I agree leaving the current behavior (using the extension.json field) makes more sense.