Apparently, they usually or only tend to add/update something when it suits situation of the game, like a patch or expansion update, at least that’s what I think.
For example: Warfront Weapons and Armors were added in BFA, as it was related with War and Faction Pride.
However, I agree with your opinion. If they add or update something now, it may look outdated compared to some next-gen hardware era graphics that could be introduced in the future. Still, I think it would be fine to add seperate, updated versions of some old mounts like Spectral Tigers and Swift Zulian Tiger, even now. Although they look outdated compared to recently added mounts, I love using my Spectral Tiger and Swift Zulian Tiger anyway.
I mean it would be extra work, but they could always update the seperately added versions later again in the future, in case of next-gen hardware graphic revamp. Like how they updated models of some important Lore characters like Lady Sylvanas Windrunner, Valeera Sanguinar and Jaina Proudmoore etc.
Also, if/when the next-gen hardware revamp happens, I hope they also add new compatible hood models that allow hair to be shown through the hood. Like Lady Sylvanas Windrunner and Valeera Sanguinar. If they add new compatible hood models, then the hair wouldn’t clip through the hood. And since they will make only these new hoods compatible to show hair through the hood. They also wouldn’t need to remodel every helmet item for every race/gender combo.
By the way, they can alter how a specific item appears on character, so I think they can do what I suggested above.
For example: there are some helmets show hair through the hood, most noticably the mail helmet from Darkshore Warfront, but the hair clipping through the hood, of course.
Appearance of some items can also be altered, depending if they are used in combination with specific item(s): Some helmets hide beard (noticed on Male Humans, atleast), forearm parts of some robes hide 3D details of some gloves, and some leg slot items alter the appearence of some boots, etc.
So, I’m always excited about news of new updates. I didn’t know about the interview that you mentioned, but if it’s correct and update of the legacy content would happen at some point, it’s good the hear that.