Now, Apple, with their iOS monopoly, managed to strong-arm the web into a position in which third-party tracking protection was finally possible. Kudos to them, now we can finally release this technology. We're doing it slowly, because by opposition to Apple, we cannot afford to break the web.
Why did Apple do this, though? If you recall, Apple's stance on privacy is quite recent. The tech world has a short memory, but I remember Apple being caught red-handed slurping data from user's iPhones not so long ago. Also, unless things have changed (honestly, I haven't followed), Apple tracks users through their native apps, because you're pretty much obligated to go through Apple's SDKs for many things, and they require a tracking ID, which brings revenue to Apple.
So, yeah, Apple's actions have had some good impact on web-tracking, and that's great. But don't forget that they do this for strategic reasons: 1/ because they want to encourage devs to use another tracking mechanism that the competition cannot emulate; 2/ to immediately hurt the competition.
Now, you are right about the double-keyed caches – although using this to track users is quite more complicated, less precise and less reliable than first-/third-party cookies, it's something we need to address. We're working on it at this very moment.
Finally, yeah, first-party cookies... hard to counter. We're also working on it (is anyone else doing so?), but that will be a tough fight.