You can never say never, but
I've never heard of a cruise being cancelled because of soft bookings.
For full ship charters yes, because of mechanical breakdowns yes. Heck ships even sail during hurricanes (they change itineraries if necessary).
As for the TA letting you know about price reductions. Some travel agents will check their clients pricing occassionally to check for price reductions. But busy TAs may find it difficult to make time for checking all their clients sailings consistently.
You can monitor the cruise line pricing pretty easily yourself online these days. Then, if you find a price drop, notify your TA, and as long as it's before final payment due date, most TAs will get you the savings. They certainly should!