I’ve thought about this as well.. even on desktop you can make the sidebar go away while reading, so having another easy way to be guided into a new topic seems to make sense. Not sure where it could be.. do you have a suggestion? Google just “minimizes” the sidebar without getting rid of it altogether, so you never lose the new email compose button.
On the other hand, in discourse how essential is the new topic button for people who don’t already know their way around? We want new members in a community to spend time getting to know the community by reading and replying to existing discussions, after all.