After that, you would lose more opportunity than you would gain through increased margins. The trick is to create the perception in the customer's mind that your service has a premium value, and that's very hard to accomplish with dialup. The problem is that the small ISP is too small for most suppliers to be interested in. It takes a lot of time and effort to find valuable services and to negotiate a deal with a supplier, assuming they'll talk to you in the first place. That is why most of us will not be successful in doing what you are suggesting, even if we had the time and the inclination to do so.
People buy luxury cars because they're status symbols.
The essential element is information. You'd have to maintain amazing user to modem ratios and have all kinds of backups in place. The bottom line is, no outages and no busy signals, period. It's quite possible, but very expensive. Many others also offer this service.