It's convenient for all businesses in that it's one less thing to worry about/have to analyze/potentially have to negotiate. The tech savvy businesses in this case (don't fool yourself into thinking that a majority of businesses can do what you just pointed out) provide a tiny subsidy in order to help improve the general health of the economy, which in turn benefits them by providing a healthy customer base and a pleasant place to live.
For large businesses that rely heavily on mail for customer communications, the solution you point out is laughably oversimplified. The difference between Chase knowing that they can definitely send mail to 100% of their customers is vastly different from them being able to send mail to 99%, especially if there's any sort of legal mandate to provide something via mail.
If you are correct, variable rate mail will find few customers and flat rate mail will survive.
Regardless, you still fail to explain how forcing technophilic companies to subsidize technophobic companies is good for the economy. Would it also be good to force companies with computerized ledgers to subsidize companies with paper and pencil accounting ledgers?
Keep in mind that that vast majority of businesses in the US are technophobic. It's not us subsidizing them, it's doing what is best for the majority. Technical literacy is not a driving factor or even particularly relevant in success in most businesses, and it's important to remember that before making pronouncements on sweeping changes that the government should enact. Having a healthy economy where conducting business is a simple and straightforward matter is something that technophilic companies should care about, since they're the ones that pay most of our bills.
Also, it's not just the flat rate, it's also the guarantee that the govt. provides of being able to send mail to everyone with an address which is important. Private mail carriers are unlikely to make the same guarantee.
For large businesses that rely heavily on mail for customer communications, the solution you point out is laughably oversimplified. The difference between Chase knowing that they can definitely send mail to 100% of their customers is vastly different from them being able to send mail to 99%, especially if there's any sort of legal mandate to provide something via mail.