> If the desirable employees, and the capitalists who want to actively oversee their investments weren't already in the more expensive areas
Most of those people were out in cheaper areas 40 years ago, when suburbia was still ascendant. So "they just happen to already be there" doesn't explain what we observe today.
> or if there weren't costs to both associated with regular travel
That is just restating why walkable cities are nice to be in: travel has costs, being close to things means less travel.
> if jobs moving to the new place wouldn't actually turn it into a more expensive, denser place
That clearly didn't actually happen when the jobs first migrated to the suburbs. The suburbs remained suburbs. The jobs were in low density office parks.