You know, illustration is the single highest expense in a book... This is not a surprise to hear. Actually, how many pictures of historical armors are public domain? I'm mostly curious about how the suits that I recall from the Met's Arms and Armor collection stat out.
Illustrations may only be necessary for some odd combinations of species and gear. Maybe some notes detailing the gear in the illustrations y'all already have?
Oh, I'm not saying we can't or won't illustrate something like this...however, we could never do it for everything. You want it for chain, but someone else won't know what articulated plate or ringmail looks like. And honestly - how many people don't know what chainmail looks like? Doing it for anything that includes the adjective "historical" only compounds expectations (and as you note, can be found on the web). If we did an illustration of this sort, it would be to show coverage and examples of no v. light v. heavy fittings - NOT any particular style of armor.
I would expect gear illustrations in Gear for the Ages to follow the same general themes of Fantasy Craft - good art representing a smorgasboard of time periods, technology levels and fantasical-ness without locking us down down to any Era, historical reference, setting, or technology type.