This graph shows the time to transfer various amounts of information. If I'm transferring a floppy, it only takes 8 minutes on POTS. On the other hand, if we need to transmit 20 floppies, the size of Word 6.0, POTS doesn't do it. ISDN still takes 30 minutes.
And so what's a reasonable bandwidth if we're going to transfer anywhere from a 1995 CD with 600 megabytes to the next standard 9 Gigabytes CD due in the next few years. You really need 10 megabits or more. MPEG 2 requires about 6 Mbits per second.
So I say 25 megabits because that's an ATM standard and that's a reasonable engineering target. Cambridge England has this in prototype form. It doesn't hurt our phone bills since telecos are spending our money on inferior alternatives.