Yes, the paywall is reasonable, I agree. I think what the OP meant by 'social contract' is that if everybody were to use an adblocker, we would end up with a mostly paywalled internet. All the sites that currently have ads, would have a paywall.

The reason why some people get to browse the internet free, and without ads, is because there are some people that don't. Hence the 'leeching' part.

The part that annoys me sometimes, is that when there IS the option to pay to remove ads, and people still use adblockers in this case. How is this justifiable, morally?

We didn't have a paywalled internet before ads became commonplace. Most content on the internet is still user-generated and almost all of those users do not get paid anything from the ads put on their works. Hosting is quite cheap unless you want to run a centralized service that serves literally everyone.

If anything is a social contract then it's that if you want to provide a paid service you are up front about requiring payment. Ad-supported websites don't much care about that social contract because they think it's more profitable to pretend to be free when they are not.