Now at that point it could go with a dropdown presenting the
choices, but let's look at how this sort of thing, with two or
more edit modes, is usually done across existing platforms: Click
the link/button, and generally it opens up to the visual editor by
default. From there, the source mode is a tab or option that can
then be switched between as needed, with the visual editor also
doubling as a preview if the user is working mostly in source.
In the long term I believe that VE will be the primary editing experience for everyone, and they may occasionally switch to a source mode.
We offered the option because today, large articles still have serious performance issues, so even minor VE-friendly edits, aren't practical when editing those articles.
It's also worth noting that, at least for now, switching between visual and source mode will be expensive on the server and take a bit of time.
I expect we will move more towards this as we go, but it's not viable yet.