I would question that page level editing is actually "most important call to action on the page"
What would you put above it?
Reading? Clicking links? Viewing images? Those are not "calls to action". They are functions people are motivated to do naturally, and which will not be detracted from if you make Edit more prominent.
In VisualEditor there effectively is no section edit, we're retaining it for consistency with wikitext and to provide extra CTAs. Other CTAs are viewing history, viewing the discussion page, or using editor-related items like notifications or watchlist. These are less important or ancillary to editing.
Our single biggest challenge right now is getting people to edit Wikipedia. We haven't made significant progress on turning that trend around, so if we're going to do a redesign of the skin as significant as this we need to emphasize this. As Brandon right points out, just turning the current Vector edit button in to a mw-ui-constructive style button would not solve the deeper problems in Vector, and would no doubt be odd-looking. I'm really glad this prototype starts to tackle those issues. But if you seriously think that Editing articles is not our most important call to action then we need to have a sit down and talk about this in person.