how is that supposed to happen with a 1020 that phone doesn't even get all the features of Denim
Because Denim/Cyan/Black and the likes are firmware updates. They include drivers, ways to manage the camera hardware and background processing tricks. That's more low level firmware and more invasive. This requires more testing and can impact the device in bigger ways.
Windows Phone (8.1 or 10) is purely operating system. And the way Windows Phone works is that it just reflashes a new image with each update. That's why it takes time to "prepare installation", which basically means extract and prepare image. During the update process your data and settings will repopulate the device and then it's done.
This is safer and easier and less error-prone than the firmware type update that Denim/Cyan brings and isn't all that device dependent if they don't change too much in how Windows works with the hardware. And they don't, as the kernel doesn't change.
The Lumia 1020 doesn't get the firmware updates (Denim etc.) anymore, and neither does the 520, 620, 920, 925, 720, and so on. They do however get the operating system updates. (confusing part is that MS does call it denim on these devices ...)
Since Windows 10 and Windows 8.1 share a similar kernel with similar hardware control and similar ways of managing storage and so on, it will end up being relatively easy to update. That's why the 1020 may not get all the fancy firmware tricks, but will get the Windows 10 upgrade. Don't forget that the Windows 10 upgrade is less invasive than many people think. The changes are not very deep in the OS because 8.1 is already a brilliant and very capable system that already supports most of the changes under the surface that Win10 will bring. All Win10 does is modify the start screen, add a few libraries, apis and some other small-ish features.
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