Agreed, but Google does have UX Signals. In a perfect world, you'd think Google would be able to identify the "wall" as part of their signals they measure from a site & rank accordingly.
Absolutely not...sure it's spammy. So use an extension for a personal blocklist by all means, but as a Quora user I like having answers readily available. Don't make your lack of creativity Google's job to fix.
/grump over
Seriously though, I know this'll be unpopular but Google already tracks User Experience Signals to identify sites whose answers are difficult to get to. If Quora should be identified & ranked lower, that's totally a Google problem, but dropping entire companies & making it Google's role to monitor sites & decide which logins are "walled" and which are just UX is a terrible idea (do you trust Google to get it right every time?).