As already stated, this is Dumb. Every iphone I have ever had got scratches on the coating. What a waste that would be of sapphire
Scratches are only the tip of the sapphire iceberg. Micro-scratches and the occasional deep scratch may be annoying to some and infuriating to a few, but a shattered screen is a heartbreaker for all affected.
As a selling point? "Fewer scratches" isn't going to sell a whole lot more iPhones - most people don't notice the micro-scratches, as all micro-scratches do in practice is reduce contrast. Up the screen brightness, and that issue is overcome (while increased brightness doesn't change contrast, the net effect is similar - easier visibility).
The key selling point is
shatter-resistance. That may affect a smaller number of users than scratching, but it's a result all purchasers fear.
Meantime, the overall thickness of the transition layer places a near-maximum on the depth of scratches, and as it varies in hardness between silica glass at the very surface and nearly pure sapphire at the base, it will be more scratch-resistant than silica glass just a short distance below the surface. The deeper the scratch, the more visible it is, so this should still improve the optical performance of the screen.
The fact that a transition layer is necessary says a lot for the intrinsic qualities of sapphire - if it can't be coated directly, it's pretty slick stuff.