LinkCache is a kind of cache of last resort for parser linking, and could be removed without much performance impact. I think marking of links to disambiguation pages (bug 8339) should be handled in the same way as stub links. LinkHolderArray::replaceInternal() should do a batch query, and the result should be passed to Linker. There is an existing GetLinkColours hook, maybe that could be used.

Ideally, the fixme comment on line 349 of LinkHolderArray.php should be fixed, i.e. clarifying the role of LinkCache and removing it from the data flow path from Parser to Linker.

The issue with storing page_props data in a global-lifetime object like LinkCache is that it would use a lot of memory. LinkCache has no limit on the number of titles it holds, and it is difficult to implement such a limit due to the way it has historically been used. LinkHolderArray, by contrast, has a limited title count and its lifetime is limited to the length of the parse operation.

Similarly, bug 17212 should be done with a batch query in CategoryViewer, similar to the treatment of the category table (cat_subcats etc.). It shouldn't be necessary to modify Linker in that case.

Today I learned of the GetLinkColours hook. That's good enough for me, and it's unclear how this bug could be fixed.