What this video covers
The clip explains why carpet reaches a point where cleaning stops being an option.
The padding underneath absorbs water and holds it against the subfloor. It cannot be cleaned effectively in place, so a musty smell usually means the problem is in the layer you cannot reach.
The takeaway
Smell is the deciding signal for carpet. If it is musty rather than merely stained, the padding is the issue and replacement is generally the honest answer.
Read the full written guide: Mold in Carpet.