I think we don't need a downvote button and as I mentioned, a limit amount of endorsement giving from an account to another per day or amount of endorsement that one account can give out per day may help to avoid the issue.
Plus, match-matching is random and everyone can be in a team with a bad player. That's a fact and I think everyone needs to overcome the problem having negative teammates since it happens to everyone. Some matches still can be saved, some matches cannot, that is just it and all you can do is play your best and not giving up. You already accept the result no matter what when you hit that finding-match button so just prepare to deal with every situation. All kind of toxic players, AFKers, intentional feeders and rage quitters deserves criticisms because of their bad effects on other players but you don't always meet them and even if you do, trying your best and not giving up then even if you lost the game, you should still be proud of yourself and maybe you will receive endorsements from others for not giving up and fought like a real paladin
My idea for endorsement is to make it various and specific like "good teamwork", "life saver", "funny", " good aim", or "friendly player". Each player can only hit +1 for one of those to another player once in a match.
There is absolutely no need for one to be able to spam the same VGS repeatedly over and over again. This is just something to get the players mad about each other and raise the toxicity. No other reason.
Two times maximum the same VGS. 4s CD, I say.
Edit: 4s between 1th and 2th repeat. To use the same again, 10s seem OK.
Ahh Warframe, when their parts and pieces system was causing clipping issues with some new skins they simply held those back till they could make the different pieces fit together properly without ruining players 'fashionframe' as we call it. If only Hirez did the same instead of throwing a big reason to buy different skins out the window..