This is a horrible idea. First, the idea is inherently flawed. Also, there are other boards, and there are people who don't play ranked. In addition, just because someone is Bronze doesn't mean they don't understand the metagame. Fourth, Riot would be alienating 70% of their fanbase. Finally, by your own logic, I have every right to ignore you.
First, it flat out wouldn't work. They can still comment, and that's what keeps a thread on top. So the "quality of discussion" wouldn't change. The upvote/downvote numbers would just be 70% smaller. So, to fix this, you say "I know! Ban them from posting if they aren't Gold!" Great idea! It still won't work! I've seen people in Gold 2 who still don't think support is a real role. I've seen detailed and accurate analysis of what a patch will do to the meta from a Bronze 5. Would you let the former post and not the latter? I think not. The quality of discussion would be unchanged either way, and if you take my "suggestion," the quantity would be drastically reduced.
Boards other than Gameplay and Balance exist. Would the same system be implemented there? Would we not be qualified to think a joke is funny? Would we not be allowed to upvote an interesting champion concept? If we would, how would Riot implement this only on select boards? It would take a lot of programing, which takes time and money away from fixing problems that actually exist. Also, what about people who don't play ranked? I find it highly believable that someone could have Platinum-level skill without ever joining solo queue.
In addition, game knowledge is not exclusive to high ELO. I understand the strengths and weaknesses of most champions (not that this means I own them all or can play them all; this just means I know what they do and how they do it). I understand that taking towers is important, taking dragons is important, and you need to ward. I may not be good at those things, but I understand the importance. People are often exaggerate balance problems (see: Vayne, Riven, etc.), but they're often right to some degree. Ekko was quite strong on release. Riven does snowball pretty hard. Vayne is a good tankbuster, and is pretty strong in the current meta. What increases your ELO is timing, foresight, and mechanics as much as it is game knowledge. Any Bronze 5 could tell you sated devourer would be a buff to autoattack junglers, for one thing. To be above Bronze 5, 25 LP, you need to understand a lot about the game. Much of it you learn by level 20. It's honestly offensive that people treat Bronze and Silver like they know as little about the game as a level 3 newbie.
Most players are between Bronze and Gold. In fact, I believe it is safe to say that 70% of players are Silver 1 or lower. Riot would be taking a huge risk to ban 70% of the playerbase from voting on balance threads. It would piss a huge number of people off, myself included. Riot would probably lose a significant number of players over it, and that is not worth maybe slightly possibly improving the quality of discussion. If you decide to ban them from even commenting, then Riot loses even more players and the boards are now utterly dead. Congratulations. I hope you're proud of yourself.
Finally, by your logic, I have every right to ignore you. Only Gold and above provide meaningful discussion, right? You aren't even level 30! How could you possibly provide meaningful discussion? Don't bring rank into things, especially if you can't back up your so-called argument.