This week in "HN looks at software that took thousands of devs 30 years to write and says 'eh I could've done that in a day!'"
"that deep" should be read in terms of the resources of a large company. It's a small fraction of the OS.
1. You already know exactly what functions it needs up front, unlike the first time you coded it.
2. You have much more advanced libraries and development environments available.
3. You have more years for the redo than the original took.
I would not call that "incredibly complicated". I would call it "doing a significantly easier version of what you already did".
And "The Control Panel isn't that deep. It's not as though Microsoft needs to create a UI for every advanced Group Policy setting." is talking about scope, not complexity. So I'll say again that it's a small fraction of the OS.
(It doesn't matter how complicated it is in a vacuum, anyway. As long as the complication is on par with the rest of the OS, which it probably is, then we already know that Microsoft can handle it.)
> similar to what Microsoft actually did
What they actually did was redo parts of it in a giant mishmash of overlapping and conflicting UIs.
Even if they need to keep around parts of the old UI for third parties to hook into, there's no good reason that the new UI is so far from complete. (Except for the obvious reason of "it wasn't a priority, they don't care as an institution that it's a horrible mess")