Players with half a thousand games with no progress are a good thing for the system, since it means at least that player has been sorted to an appropriate mmr for their skill level, which in turn makes it easier for other people to be sorted to their appropriate skill levels.
If someone with a 'true' skill level of 1500 mmr is currently at 1200 mmr with a bunch of people also at 1200 mmr, you'd expect them to carry, and they almost certainly will if everyone with 1200 mmr in that game has a skill level associated with 1200 mmr. The problem is thats unlikely to be true unless they all have hundreds or even thousands of games played, so the 1500 mmr 'true' skill player might very well lose the game because the opponents were actually 1600 mmr skill players who hadn't reached their proper mmr yet.
Also, you seem to be sometimes using elo to refer to rank rather than mmr. MMR is the only thing that matters, and rank is a purely cosmetic overlay that has no impact on anything except you you can duo with and what end of season rewards you get. It's true that for the majority of players who don't have enough games played to overcome variance, that their rank does not represent their underlying skill, but that isn't really a problem, since it's just cosmetic and doesn't actually impact matchmaking. If riot wanted to make ranks actually be statistically meaningful representations of a players skill level, the solution would be to have 1k placement games rather than 10, not to limit the number of games played which could only make things worse from a statistical standpoint. The problem is most people don't have the time or desire to play 1k games in a season, and since ranks don't matter, riot gives us ranks much sooner so people can have a sense of progress and they can be just a bit of fun.