At what point does the AI buildout become a balance-sheet trap?

Big Tech keeps spending as if future AI demand is already guaranteed. Maybe that bet works. But the scale is starting to feel strange. These companies built some of the most profitable, asset-light businesses in history, and now they’re racing to turn themselves into infrastructure companies with enormous ongoing costs. The usual answer is that demand will eventually catch up. But what happens if AI becomes genuinely useful without becoming profitable enough to justify all of this capacity? That seems like the part missing from most discussions. The technology can be real, widely used, and economically valuable while the infrastructure buildout around it is still badly overextended. So what would actually prove the spending is justified: revenue, margins, utilization rates, or something else?