When we offer something to users we should always offer them with choice. We cannot say to user like this is the new feature that i have introduced/discovered. A part of people will accept and a part of people will not.
Gmail team earlier they have given this feature by default. Ie., Merging the conversation with same subject line. Users are not given any other option. Finally we are used to it. Now considering everyone they have redesigned this option by leaving the choice to user to either turn it on or off.
If you are the one who doesnt like the feature that was existing for years then you can follow the below steps to disable that.
1. Goto Settings --> General --> Click on "Conversation View Off"