Getting updates from Windows Update with everything they include, was also part of the deal.
As you say, they have responsibility to their users;
If the URL handler was old Edge, and they didn't change it while removing old Edge, the user would have broken internet access and not understand why. That's no good.
If the URL handler was updated to quietly move to new Edge, they just swapped out a major piece of software for one which looks and behaves differently, with a different icon, without any introduction or warning or confirmation, which is an awful experience for users. Microsoft get accused of special-casing themselves and their browser, and it breaks the work Microsoft has done for years to stop programs changing file/protocol handlers without confirming it with the user. [see Raymond Chen writing about it here https://devblogs.microsoft.com/oldnewthing/20181016-00/?p=99... ]
If they decide a browser is not part of Windows and you have to install new Edge from the app store yourself, tens of millions of computers are now stuck on the last version of old Edge basically forever - because there's no Windows 11 on the way to replace it - and it becomes the new IE6 millstone for web developers. If they also remove old Edge at the same time, millions of people lost their browser and don't know why - and the only default one remaining is IE 11.
There is a new thing, the hypothetical user who had settled into Edge does need to know, however it got installed.
As you say, they have responsibility to their users;
If the URL handler was old Edge, and they didn't change it while removing old Edge, the user would have broken internet access and not understand why. That's no good.
If the URL handler was updated to quietly move to new Edge, they just swapped out a major piece of software for one which looks and behaves differently, with a different icon, without any introduction or warning or confirmation, which is an awful experience for users. Microsoft get accused of special-casing themselves and their browser, and it breaks the work Microsoft has done for years to stop programs changing file/protocol handlers without confirming it with the user. [see Raymond Chen writing about it here https://devblogs.microsoft.com/oldnewthing/20181016-00/?p=99... ]
If they decide a browser is not part of Windows and you have to install new Edge from the app store yourself, tens of millions of computers are now stuck on the last version of old Edge basically forever - because there's no Windows 11 on the way to replace it - and it becomes the new IE6 millstone for web developers. If they also remove old Edge at the same time, millions of people lost their browser and don't know why - and the only default one remaining is IE 11.
There is a new thing, the hypothetical user who had settled into Edge does need to know, however it got installed.