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
The major reason for tokens to lose value is because simultaneous dumping from bounty participant, they get only little part of the total token but that little part is the major part of the circulating supply, that's why the effect of hunters dumping is always more effective negatively.