Why other powers remain unlikely capital-ship builders

Would powers outside the five treaty navies build battleships or battlecruisers in this scenario?

Probably not; states without an existing capital-ship industry would have to order abroad, and few had a strategic need or budget strong enough to justify doing so.

Britain, the United States, and Japan begin with active yards, specialist workers, and programmes already in motion. A smaller navy would face the much higher cost of creating that ecosystem or buying a ship from one of those powers. South American competition is unlikely to restart at the required scale, and dominion ships would still consume British resources and treaty allowance. Transferring Renown or Repulse to Australia or New Zealand is more plausible because it redistributes existing ships rather than creating a new industrial competitor.