There's context lists and project lists... but there's no "Do This Project and Nothing Else if You Want to Advance your Life And Career" list.
IMHO you've missed a point... YOU decide what goes on your context and project lists.

If you sit down at your weekly review, and decide that "this top priority project" is the one thing you want to move forwards between now and the next weekly review, then the actions related to that project go on your context lists.

Everything else can move to someday maybe, because you've decided that you don't want to spend any time on it at the moment.

You shouldn't have to be reviewing your life's purpose and goals every 5 minutes - that's a job for the weekly review.

In reality, there are probably multiple things that still need to be on your action lists, in addition to your top priority project. The world still turns around you, however focused you are.

Like you, I focus on one main project at a time. I keep my lists short and don't hesitate to move stuff to/from someday maybe constantly. If it's a short project, actions for that project may be the only thing on my context lists. More often, however, projects cover a longer time frame and there are still other small projects I need to keep moving forwards at the same time. I still need to buy dog food and pay the bills, however important the main project is. GTD handles all that 'stuff', freeing me up to focus on 'the project'.