Google knows people that dislike changes are always louder than people that like changes. It doesn't mean they are in larger number. So no matter how hard you scream, don't fool yourself into thinking companies take these change decision lightly. The article claims it lack of UI tests which is an ignorant assumption.
The goal of the new compose is very clear. Allow you to write an email while still reading others. It undoubtedly completed the goal. Of course there are some drawbacks like having to remove fields that would be visible by default otherwise.