Microsoft spent $2.5 Billlion for Mojang. They may not work on the JAVA version directly but I guarantee they influence it's overall market path. Based on their behavior with Windows 10... I'm expecting ads at some point in the future.
Once again, as has been stated multiple times, Microsoft has not, and will not, have any part of the process of developing and maintaining Minecraft: Java Edition (apart from the name change). So IF there ends up being ads in Minecraft: Java Edition, that will have been a Mojang decision, not a Microsoft decision.
Microsoft can do whatever they do. It is their game. If they get feedback on forums, where people post comments, some of it strong feedback, then possibly they'll allow players who feel passionate about a new UI feature to have the option to turn it off. I don't like the direction this leads, not having control over UI notifications I am seeing. What next? Little advertisement messages I can't control? It is creepy.
except microsoft doesn't do anything with java minecraft. That's still (unfortunately) left to the morons at Mojang - the guys who pushed launcher 2.0 and the combat update, among other things.