If dumpers can dump coins and tokens in a reasonable way it won't affect the value much , lose in value when dumping happens if a dumper dumps huge amount of the token at once , this is a wrong way to dump coins, this is why till date some devs put locks on their tokens ,some even release percent by percent on term basis just to avoid huge dump.
The best way to dump is not to dump in an instant or a whole at once
Real talk, you can't "make" someone to dump or sell their bag in your "reason way" that dump little by little and not all in one go. Even devs try to lock or force their way into people's bag, they can't prevent the inevitable.