I agree with all you say (except for the remarks about Nikolai Demidenko, as I've never heard of him before).
And the most unfortunate problem with Medtner's popularity (or lack thereof) I experienced first hand when I casually listened to Hamelin's recording of the Sonata-Triad, Op.11 for the first time. I found the music "unremarkable" and didn't even consider it again for a year or 2. But this year has been a busy one for me in regards to discovering (un/little)-known composers, including Feinberg, Busoni, Medtner, several other Russians, and particularly the great Scottsman Erik Chisholm, whose sheet music is very hard to obtain, and very expensive when you can.
But back to Medtner. A friend of mine knows Marc-Andre Hamelin personally (as well as Fredrik Ullen, Carlos Grante, Sorabji (near the end of his life) and many other influential composers and performers), so when he suggests I try listening to an unfamiliar composer/work, I pay attention. And when he told me that, when asked by Hamelin what one piece of music he'd take to the moon/desert island, my friend said Medtner's Op.25/2 "The Night Wind" sonata (which Hamelin had just released). So that inspired me to find out what was in this music that I never heard before. And with repeat auditions it all began to make wonderful sense, and now I have no problem accessing a new Medtner work, because I've learned how to crack the shell of complexity that keeps the true beauty of the music hidden from most casual listeners. This really, unfortunately, seems to be music for pianists far more than general audiences. Despite the high degree of perfectionism in every work ("every piece is absolutely perfect" as my friend emphasized), the elements that make music memorable, and ultimately popular, such as lyricism, powerful harmonies and rhythms, etc. are generally tightly confined in Medtner's works. And until one grasps this concept, the music will remain heard, yet "undiscovered". And when you realize that one of Medtner's closest musical rivals at the time was Rachmaninoff, who's music rarely lacks all the aforementioned elements, it's no wonder which one was more popular. Like it or not, Rachy's melodies and harmonies are right there in your face (ears

). So who's music is the public going to remember, discuss, share (through performance, as the recording era was just getting started)? I doubt even now, as in the 1920's, no one attending a Medtner recital would remember any particularly beautiful tune or heart-tugging harmony once they went home. They may recall the music being difficult, sometimes quite technically brilliant, etc. but they would hardly have
discovered what they had been listening to, and for many, never would. And as you observe, in these days of urgently needing to text someone, or follow the insipid masses on Twitter, or decide which of the 1000 TV channels to watch tonight, who but a serious pianist or devoted music lover is going to take that extra bit of time to crack the shell that hides the wonders within?
Celebrating lamentations, I remain

Lontano
PS: I have a friend with a 17-year old son, who recently reported that he had made 7,000 text messages in one fairly average month. Fortunately his dad pays for a plan that has no limits for texting. He has a 20-something daughter who has a friend in Canada (we're in the US) who has to pay
long-distance charges for all her text messages to a foreign country. Hey! I may be getting older, but not stupider (I hope). What the hell is going on these days? The collective cell phone designers and service providers have created, in a very short time, something that has suddenly become a critically urgent need for millions of people, who just a very few years ago never had, nor really needed; today, despite the great recession, paying for texting on your cell phone seems to be the last thing most younger people would choose to give up.
Brilliant!!!