Based on what Greg said in this AMA, it’s clear that Roblox wants to let users have control over whether heads are static or dynamic. So I’m not sure why classic faces should be left offsale if this toggle would be added in the future.
The versatile approach for this update would be to let players buy classic faces, and decide whether they’d like to have them dynamic or not, and vice versa. This would allow for players to explore the dynamic heads feature, while not being limited in what style they’re going for.