Yeah, that sounds like a great feature. The thing is though, that feature isn't going to go away just because PWAs are a thing. The only difference with PWAs would be that sites would _also_ have the option of building offline support into the application itself rather than having to rely on one specific browser to handle that feature for them.
Not to mention PWAs would allow for more complex offline features Safari doesn't support, like syncing the front page of a news site and all associated articles, or automatically downloading new articles as soon as they're posted.