Cumulative update won't install (KB4571756) (Error 0x800f081f)

This is correct, in a way. Windows 10 uses "Cumulative Updates". Each Cumulative Update includes everything from all previous Cumulative Updates. With earlier Windows versions, such as Windows 7, a clean install would require many, many updates to bring Windows 7 up-to-date. This often took many hours, or had to be done over a few days to finally catch up.

With Windows 10, you just need to install the latest major version (comes out twice per year) and one Cumulative Update. Then you are up to date. There are other updates, such as .NET updates, or Windows Security definitions that come along also. But even these are "Cumulative Updates" in each category and include all previous updates to the major version.

As each Cumulative Update arrives, previous updates become "superseded". It is possible to view and even remove these superseded remnants of past updates. Doing this reduces the size of Windows by removing what has been superseded, but it also makes it impossible to remove the last Cumulative Update, if it is causing problems on your device.

So if a CU introduces problems on some devices, those same problems will be carried forward into the next month's CU until Microsoft finally works out how to resolve the issue (without creating more problems for other devices in the process).

No matter how big a problem might appear to those of us that have it, it could actually be only affecting a tiny percentage of all Windows 10 users. There is an incredible variety of hardware combinations out there and Windows has to somehow work for all of them.

The current Version of Windows 10 is Windows 10 Version 2004. With the next update, they have changed the numbering to "20H2". Rather than trying to indicate a specific month in the Version number, it now indicates "H2" as being the second Windows 10 release for year 2020. The build number will change from 19041.xxx to 19042.xxx once the "enablement package" has been installed. This will activate the 20H2 features that are currently installed, but in a dormant state.

I tend to agree that in place upgrade or clean install does succeed sometimes.

Trying out the October CU ( cumulative update ) there seems to be a pattern emerging.

It all started with the the August CU KB4566782 failing to install and trying over and over again. So I stopped the updates and awaited September patch Tuesday.

What happened then is that  KB4566782 disappeared and KB4571756 repeated the failed to install behavior. By then I found out how to hide a failing update.

Guess what happened at October patch Tuesday : KB4571756 disappeared and now 

KB4579311 is failing to install in exactly the same way as the 2 other before.

Educated guess : there is some **** code that slipped into the August CU and is now being carried over to every monthly new(?) CU.

Question : why does Microsoft not fix the **** code in their CU deployment instead of forcing us into a new Windows installation, which is still no guarantee to fix this issue.

So for now again hiding KB4579311 and awaiting 20H2 before downloading iso image for in place upgrade ( with hail Mary before...)

The in-place upgrade hasn't worked for everyone. Some are reluctant to try a clean install. There is no CLEAR solution to this because we don't even know the cause. Those who were able to fix the issue with an in-place upgrade kinda got lucky.

So 20H2 isn't going to be a big update? Well that reduces hope for people whose in-place upgrade is failing. If I'm not mistaken, no one has been able to figure out the root of this issue? Because that might hold the key to resolving it.

Many of us affected by this issue have SUCCESSFULLY RESOLVED IT COMPLETELY by using the ISO reinstall method. It works. It also kept all my apps, settings, and DATA, then caught up on Updates I had been MISSING. 

I wasted many hours and weeks trying many forum suggestions but this one worked perfectly. I hope you all can get back to "stability".

So 20H2 isn't going to be a big update? Well that reduces hope for people whose in-place upgrade is failing. If I'm not mistaken, no one has been able to figure out the root of this issue? Because that might hold the key to resolving it.