If I understand correctly: $wgDisableHardRedirects is either all or nothing, $wgRedirectSources is only about incoming redirects, $wgInvalidRedirectTargets is only about special pages.
Enabling hard redirects for everything (i.e. for all local interwikis, cf. bug T40352: Categories should list threads, not summaries) is or can be too much. For instance, all Wikimedia projects are “local” to each other, but it's not very appropriate to automatically redirect a visitor, say, to a Wikipedia in another language.
It would be better to allow whitelisting for specific local interwikis, which could be used to enable hard redirects to specific “particularly close wikis”; and maybe also for specific special pages, if one wants to use a specific special page (maybe added by an extension) without worrying of all the additional possible special pages (as with $wgInvalidRedirectTargets).
See Also:
T14306: Implement MediaWiki Help Repository (fetching Help: pages from MediaWiki.org or Meta, similar to file description pages from Commons)
T45591: Link translatable help pages on mediawiki.org from the relevant special pages, actions etc.