It's already been done. Wired, for instance, only shows you the first paragraph of a story until you've made an account and logged in, even in reader mode.
If you wanna split hairs, sure. "Breaking reader mode" is a component of an access wall. For a while a lot of sites didn't have that, so reader mode would work to skip all that.
For sites dependent on link-aggregator sites (such as HN, Tildes, Mastodon, Diaspora, etc.)[1] for spreading and sharing content, hard paywalls tend to very strongly discourage further readership and sharing.
I'm aware that HN has multiple levels of penalties applied to sites / domains, for various reasons. Users may also flag inaccessible content. (I certainly do.)
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Notes:
1. If you've determined that "etc." is doing much heavy lifting here, congratulations. The resources listed are highly encouraged.
Not necessarily as they still want to be referenced properly.
The workaround was to whitelist Google's spider ips and remove the paywall for those but with the progress of other search engine / social nets it's more complicated to paywall without hurting SEO.
