i agree with others that on a business ticket you usually have to make sure that you will arrive in good shape .

usually C for business is paid so you are ready to work.

anyway i would accept a downgrade under the following cirumstances:

1.) F seat on the same flight is not available. usually i would ask for an opup to F if they want my C seat.

2.) if F seat is not available for upgrade , than i would ask (if i have time) to be put on another Staralliance fight in F on the same day + compensation depending on how long i have to wait (if it is more than 2-6 hours i would ask for 600Euro compensation + F)

3.) if i dont have the time to take another flight or fly the next day etc. i would ask for the difference C to Y in cash to give it to my employee.
+ i would ask for another 600-1000usd compensation for myself in VDBs + getting a exit row seat or a whole row blocked for me.

i am usually very happy if i get bumped on Thai or Lufthansa as these two airlines pay the most money .

last time on TG i got for a downgrade to C from F (which is very rare) 650USD + a one time upgrade for one class on any thai routing.

if you are standing at the counter there is often not enough time to think, so write it down how much you want in case of possible downgrades , and if you get bumped tell them your conditions.

if they accept good for you, if not you are in the seat you booked and everything is good too.

dp