> Do networks not fill up and get congested?
In theory they can, obviously. In practice if you pool 100's of users, the "regression towards the mean" kicks in with a vengeance and it almost never happens.
How do we know this? Well in other jurisdictions like Australia say (which is where I live), ISP's are required to guarantee a minimum peak period bandwidth. If their customers don't get it, they can complain to the government and _the government_ will prosecute the ISP.
The ISP's squealed like stuck pigs when this rule was implemented. What actually happened when it was introduced was nothing, apart from a few prosecutions when the ISP's lied. When I say "nothing" I don't mean absolutely nothing as every ISP had to label their wares with the real bandwidth and cap they would give you. For example, if you had a nominal 100Mbps internet link (which is the number they always quoted before the rule) an expensive ISP might say it was 95Mbps guaranteed. The cheap ISP, surprise surprise, might say they only guarantee 85Mbps on that same link. But no prices changed, and mostly no speeds changed. "Mostly" because a few liars had to stop lying and once the truth was visible the were driven out of the market.
Economic textbooks tell you a well informed market functions better than when there is an information imbalance between buyer and seller. All Australia did is correct that imbalance in the internet market, so it functioned a little better at matching buyer to seller.
But Australia went much further than that. There are no ISP monopolies in Australia, so there _always_ many alternatives for every retail purchaser. I won't going into how they achieved that, but I will tell you one consequence you might not expect. It means there is no need for net neutrality in Australia either. An ISP might advertise Netflix gets preferential treatment, perhaps Netflix traffic doesn't contribute to caps. That's all fair, but if an ISP penalised a customers Netflix viewing because Netflix didn't pay them a bribe - well churning to a new ISP is just a few clicks on a new ISP's web page. You don't have to ring up Comcast and plead your case - being able to change ISP's without contacting the old one is the law.
Australia solved it's internet delivery problems, including thorny problems like net neturality using the most basic of capitalist tools - putting systems and laws in place to ensure there is a highly functional market. It is remarkable how well capitalism works. It's amazing to many of us that the USA seems to prefer crony capitalism instead.