I agree that art support is very important.
Firaxis never finished mod support for 3D graphics for Civ V, what they did provide was incomplete, broken and unexplained. If we'd had Nexus Buddy 2 in it's current state back in 2010 there would probably be a lot more 3D art for Civ VI and a lot more 3D artists would have made the switch.
Civ VI can't be worse in terms of 3D support and may be better if XCOM 2 graphics mod support is any indication. Also some of
the quotes sound promising:
"A lot of the tools will be similar to what we had in Civ V. [Even] if you're a highly skilled modder, it was extremely difficult to work on leaders, and our elite units in the game, so we're paying special focus to that." I presume this must refer to modding the graphical side as modding the gameplay of leaders and units is not that difficult! As I suggested elsewhere something like the character customisation from XCOM2 for making 3D leaders would be awesome!
What is really needed for units is a way to export as well as import graphics so that new models can be rigged to existing animations or we can adapt existing models. Unfortunately I think Firaxis will be highly unlikely to provide this, but we'll see. They have created a brand new engine which could be an issue. If there graphics format is something unique/exotic then there won't be any tools for importing or exporting it unless they provide them or the modding community hacks them. We'll find out more in a few weeks!