> what happens when ALL the stocks have been bought back?

This can't happen in practice. It would require the company's value to fall below the buyback amount, which is itself a fraction of the company's cash and thus value. (Like, yes, I could engineer a weird failing company where this could happen. But that would just be describing a peculiarity of how the company failed. If the company is doing fine, this doesn't occur.)

If you have trouble with a public-market buyback, consider how tenders are done in private markets. You're a shareholder who has the option of selling back your shares. It's a direct way for you to tap the company's treasury as a shareholder. The company's shareholders could vote to distribute all the cash and assets in a buyback. But we have a word for that: liquidation.