1. The key phrase here is "build-enabling uniques."
Imagine if Shav's didn't exist, and instead "Chaos Damage doesn't bypass Energy Shield" was a prefix which only spawned on Scholar's Robes.
If this were true, droprates wouldn't need to be very high to get a basic low-life-enabling chest; even self-found could do it reliably, before even reaching Cruel, with just a handful of Alterations. However, perfecting that chest, getting it to the same power level of Shav's, that would border on a Mirror-worthy item. (Technically, Mirror Scholar's Robes would be like Shav's with better resists.)
The way I see it, unique items are bad design. They put build-enablers into a terrible binary: not enabled, or best-in-slot. There's no room for gear progression in that gear slot under such itemization.
2. This is only true once the no-lifers hit a lack-of-new-content wall (itemization is content). Until then, the gap follows the same rule as upgrade frequency; higher standards cancels out better items. Also, hitting that content wall is kind of a nightmare scenario from a dev standpoint.
Trade does the same function better. The more players there are ahead of you, the greater the supply of hand-me-down upgrades, therefore the cheaper they are to buy.