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
If the project has no future and the dev themselves are dumping their token, you will also in a hurry to dump the token, dumping token is a sign that investors are losing trust on the project, and you don't want to be late in dumping a shitcoin, it's nightmare seeing that you cannot sell your token anymore.