Innovation in any field addresses the changing needs or wants of the times.
I like quality old stuff as much as the next guy but there are good reasons why things are the way they are. Lack of innovation is not one of them.
Any "good reasons" may not be at service in search of the very best in audio reproduction, but seems more a consequence of how to work around obstacles that are also associated with convenience, size constraints, consumption issues, design demands, etc; oftentimes status quo is the desired goal, if it even is. I’m not saying there isn’t innovation at play here (no pun intended), on the contrary, but to simply bow to this kind of innovation as "the good reason" is to potentially shortchange the goal into audio reproduction and its further developement, as I see it.
I can’t speak for johnk, but perhaps part of what he finds "has been forgotten" may be addressed in Robert Harley’s review of Magico’s statement product, "Ultimate" (the only horn speakers in their product range):
Clearly, loudspeakers are a major source of detail erasure. It’s easy to imagine how large and complex power amplifiers, which must convert a low-level incoming signal to huge voltage swings backed by hefty current delivery, scrub off a bit of the signal’s finest information. It’s even easier to imagine how the conversion of electron flow in the voice coil into magnetism, the conversion of that magnetism to the large motion of a relatively massive diaphragm, and the motion of the diaphragm itself cause the smallest and most fragile components of the signal to disappear or become attenuated, while the more robust signal components pass through relatively unscathed. But it is precisely these micro-aspects of the signal that contain that last bit of information we need to identify the sound as being live rather than a reproduction. A musical signal reproduced through a horn-loaded system undergoes an identical process, but on a much smaller scale. The compression drivers’ extremely powerful magnets require only a tiny fraction of the current of direct-radiating drivers to produce their miniscule diaphragm excursions. It seems intuitive that this roughly ten-fold reduction in electrical and dynamic forces allows the process to be performed with higher precision.
http://www.magico.net/images/Reviews/Ultimate/MAGICO_Ultimate.pdf
It’s worth noting (through the remaining review) Mr. Harley’s impressions of the Ultimate system to "trespass" the line from reproduction into a live event. This may not be an exclusive claim, nor is named million dollar speaker system representative of all horn speakers, but you nonetheless feel his admiration of something that pushes the boundaries of audio reproduction. This mayn’t be innovation either, but it’s a refinement/evolvement of horn principles founded many years ago, and ones you would wish explored more widely as well as economically accessible (certainly compared to the Ultimate system).