It can't because the people most likely to pay are also the ones worth the most to advertisers. Thus you need to charge more to make up for it, thus selecting for even more valuable people, etc, etc.
>2) allowing users to edit tweets which people have been begging for for years.
Due to the high possibility for abuse either by users or people taking over their accounts this is unlikely unless it resets all metrics on the tweet. At which point you may as well make a new tweet. Same reason you can't edit the title of a reddit post.