When a question receive from the moderator the tag [status-declined] then, it automatically do not decrease the OP reputation anymore, but the downvotes continues applying.
There are some massively-upvoted status-declined posts too, you know. Presumably you don't want to stop folks from benefiting from a well-received idea that's been declined - after all, some of them have been implemented eventually, changing that status to completed.
Why should you be immune from the consequences of the community's dissatisfaction with an idea just because it's currently marked, "won't implement"?
Keep in mind, the difference between a well-regarded request and one that's heavily down-voted can be as simple as a failure on the part of the requester to explain why the requested change would be beneficial... I think we should be able to keep encouraging such requesters to improve their requests.
The reputation penalty for down-votes is not that high, but it is supposed to sting a little bit - ideally, folks try to put sufficient effort in to writing a good request, but if they don't then perhaps a little reminder isn't such a bad thing. If you can improve your request, then the rep benefit from the resulting up-votes will more than make up for it...