If somebody owns the copyright to an article, and has it behind a paywall, they probably won't look kindly on SE posting it as an attachment in one form or another. In addition, such a change would have to be implemented SE-wide by the developers. Note further that it wouldn't be relevant for most sites. So, while I see your point, I don't think this is going to happen.
If you need to refer to an article, I would say post a complete citation and a link, even if it's to a paywalled site. Many users here are affiliated with universities, and can access such links. If there is a short excerpt from the article, you may be able to quote it as 'fair use' (although, there's some ambiguity, as laws differ from country to country, and it isn't clear if US laws should apply--where SE is headquartered, or if those of the country of the poster, or the owner of the copyright).