Why should those limits be defined by one member of the team's gold and item slots?
Why should only one person buy wards/sightstone? Everyone needs to buy wards and if you are going to have the advantage of vision you should have to pay for it. Having one person do the warding is a terrible idea, and the fact that you think that it is the supports job to ward is a bit worrisome.
... a ward placed one place can't be placed somewhere else and if you use them all in bad places, you have to wait X seconds to have another charge.
You just described the upgraded trinkets as they are implemented now. That is exactly why the recent trinket changes are amazing. They supplement the teams warding, but cant be used exclusively to get good ward coverage.
Ultimately, the effect of having vision cost gold, specifically, is that people who do more with vision can therefore do less with other things that gold is useful for.
That's the point, there is a trade off for vision, that makes it a meaningful decision that you have to make.
Gold is important enough that you don't want too many people on the team to sacrifice their gold, but vision is important enough that you do want at least one person to do it;
Everyone can afford wards, and it is important enough that everyone needs to contribute. 75g isn't going to break the bank for anyone, and everyone can afford at least one pink.
I don't understand why allowing/forcing one person on every team to do that is good for the balance or fun of the game.
Everyone is allowed to buy wards. No one is forced to buy wards for the entire team. You are limited to 4 wards on the map by each player, and if only one person is warding that is not good for the game because you need to have more wards down.
The design of wards as items that take up item slots and cost gold goes back to Warcraft 3. Maybe it's time to rethink it.
They were rethought and that is why there are trinkets, and why the upgraded items are so good as to be a must buy.