The reason why the submit button is removed has very little to do with how much space it takes. The actual reason is that you can’t submit a topic with an invalid title or without tags (if they are required).
In the default composer view, you’ll get a very clear error letting you know what’s wrong:
When the composer is in full-screen mode, we hide all the fields except for the body of the post.
This is very intentional as one of the goals of the full-screen composer mode (all the way back to when it was a theme component) was to create a distraction-free minimal interface.
Adding the submit button to the full screen composer like so
is possible, but it can lead to confusion in case the user hasn’t filled in the required fields like the title (and tags in some cases).
Even if the button is visible and you click it, the post won’t be submitted. So you end up with what looks like a broken button, until you exist the full-screen mode. You will then see the error on the title / tags fields (see the first image above).
So, if we add the button, we would then also need to add all of those fields in the full-screen mode as well. This ends up looking a lot less minimal and more cluttered than what we want it to be.
Besides, using the full screen composer implies that you’re probably writing a lengthy post, so it’s a good idea to check if anything has changed in the discussion (if it’s a reply) rather than directly submitting the post.
I’m not saying that there won’t be improvements in future iterations of the feature, but for now, the keyboard shortcut is your best bet.