-It would be technically possible, but probably not make much financial sense. Each FM channel would still require as much power as if it was operated separate

-It would be technically possible, but probably not make much financial sense. Each FM channel would still require as much power as if it was operated separately to give adequate coverage; so rolling [n] channels into one SDR exciter, you'd need a power amplifier with [n] times the output. And that is only the beginning of your probl... eh, challenges. This monster amplifier would need to let go of one of FM's greatest advantages - the ability to use high-efficiency, non-linear amplifiers for the final output stage. Now that you have multiple carriers, you would need to ensure the amplifier operated very linearly (and, in practice, at much lower efficiency) to keep the various carriers from interfering with each other. Also, you'd probably be unable to use the same antenna for all channels, unless they were very closely spaced - or were all meant to cover the exact same area - and then you would need to apply extensive, expensive filtering to separate the carriers before feeding them to different antennas. (Which would be much simpler and cheaper to do just after the SDR, since the signal there would be very low power - but then you could use separate power amplifiers and make it even easier on yourself. (At which point you might as well use separate exciters, too - and be back where we are today. :))