Extremely well informed perspective. Although charge rate is still a valued concern when it comes to these devices, complicating it a bit.
That's correct, but the current charge rates are generally considered quite adequate, so it's presently a second order variable compared with the screen.
There are also two issues (maybe three) that actually provide downward pressure on the rate of power provided by the power supplies.
They are:
- Adaptor size.
- Battery capacity.
- (possibly) Safety concerns.
Adaptor size should be self-explanatory, but Apple does like to make things less annoying to the user, and bulky adaptors are more annoying.
Battery capacity is something that not everyone is aware of, but there's a well-studied dynamic where the faster you charge a Lithium-based batteries, the more it is damaged, leading to decreased battery longevity. I recall that the basic tradeoff occurs around 1-2 hours for a full charge (so, whatever the watt-hour the battery has, charging it beyond 0.5-1.0 of that wattage will begin to significantly adversely affect the battery itself).
Currently, the iPad is at almost exactly 0.5, which is perfect for preserving the battery, while still maintaining a healthy battery for a reasonable lifespan.
Going from 12W to 17W is still within that range, but I'm not sure that that's something Apple wants to cross into without some other reason. Specifically, a larger battery. But that's really just speculation. It could even be simply to make a charger that can charge and power the iPad at the same time.
But, historically, the charger has scaled in power with relation to the size of the battery.
And safety concerns is something I'm not very certain of, but heat and current are two factors to consider, though at 5V, I don't think current matters much.