Swollen, water-damaged cabinet under the sink — repair or replace?

Once MDF or chipboard has swollen and gone soft, it's structurally done — you can't dry it back to strength, because the fibres have expanded permanently. But you almost never need a whole new cabinet:

1. Fix the leak first and let everything dry fully — otherwise the new panel swells too.
2. Replace only the damaged panel. Under-sink damage is usually the base/kickboard; a carpenter cuts a new panel to size and swaps it. The carcass sides and door are often fine.
3. Seal and protect the new panel — edge-seal it and add a cheap under-sink drip tray so a future leak doesn't repeat the damage.

If you're replacing anyway, use a moisture-resistant board under the sink. Repairing one panel is a fraction of the cost of a new unit.