My son is a big fan of the Java Edition, because it has the most options and allows the most modding. But some of his friends have the Bedrock edition.
If this change means that you can use the same account for the Java Edition and the Bedrock Edition, that would be great. The Bedrockers could move to the Java Edition with the same account, or Java Edition players could switch to Bedrock for the occasional game with Bedrock players without losing their access to the Java Edition.
If that's the case, then this is awesome.
On the other hand, I'm not a big fan of Microsoft Accounts in general. Microsoft pushes them too hard on Windows 10 users, and for the most part, they're pretty bad. Overly complicated, mostly, and 9 times out of 10 it's not what you want.
I even made the stupid mistake to use Microsoft Family accounts to keep an eye on my son's gaming habit. Microsoft Family is probably the worst thing Microsoft has ever created. It's overly complicated, the UI is awful, it simply doesn't work, while pretending to work, and at some point my son's account got locked with no hint of how to fix it, except create a local account for him.
All of that has nothing to do with Minecraft, but it explains why I'm wary of Microsoft accounts.