actually, i'd say it's more like closing up shop after 200 sales because people who already purchased their item started lollygagging around the store doing random things unrelated to the store, causing other customers to not be able to make their purchase
and you can blame that thread you linked to getting locked so soon on the people who posted what they already answered in the poll... theres no need for people to reply with just "PC", "Xbox", or "ps3" and nothing more then "because my XX is junk"
In that case, you just remove the troublesome people from the store. That happens here, if anyone kicks off, the post gets reported and the mods remove it. They're really good at keeping topics on track, and if it does spiral out of control, the topic is closed after being reviewed. That makes sense and is something I totally agree with, and is something the mods here are really amazing and efficient at doing.
If you have a poll, you're going to get posts like that. I wouldn't call any post meaningless unless it's utter spam. People mentioning why they use their platform of choice, what platform they'd like to use, if they're planning on keeping to a console/PC on other games or the next generation. Sure, it's not the most thrilling conversation of all time, but it's certainly not meaningless. It's going to be popular, it's going to have a lot of replies. The topic shouldn't be punished just because it's popular.
You mean like a closing time?
Closing time is because it's not financially viable to keep the store open all day. How many people are going to want to buy clothes at 3:30AM? Not many.
If it is financially viable to keep the store open 24 hours a day, then they do. Like large supermarkets. They have enough people coming in in the early hours to make it viable to keep it open.
Even if a shop closes, it opens again the next day. People aren't forced out of the shop just because there's been a lot of people there that day. They don't go "right, we've served 200 people, everyone out".
Forums are constantly open, they never close, they're always available. It's not like closing somewhere because it's over capacity, that doesn't happen on a server operated by a huge company. Putting an arbitrary limit on topics just defeats the point of having this freedom.