In my opinion, it's not a really fruitful discussion. The main problem is that people seem to think that the quality of music is somehow related to the musical form which is nonsense. For the first 1600 years of the common era (the year of the first opera performance), there was no "l'art pour l'art" music in the modern sense. Major works of the time before that had no "catchy" melodies and could probaby be passed of as 20th century music, e.g. Messe de Nostre Dame by Guillaume de Machaut [1] and let's not forget that after that, 99% of the music produced for the next 300 years was mostly inconsequential just like 99% of the music produced today is inconsequential. When talking about classical music, we are talking about maybe 50 actual people that were unusually gifted. JS Bach improvised 3 voiced fugues on the spot on practically any given theme. Mozart reportedly memorized many-voiced chorals from one hearing. Beethoven composed his greatest music while being almost completely deaf.

All those greats were writing in the popular forms of their days which constantly changed. There were not that many great "pure" fugues after Bach, not that many great piano sonatas after Beethoven. So why should there be so many great symphonies or string quartets now? Every form has it's time and it's place.

There will be a couple of great composers with similar skill as the "classics" left from the 20th century but just as all the great composers before them, they will have taken the popular forms of their time and enriched them with layers upon layers of musical complexity to be discovered (and enjoyed!!) by the generations to come. I believe McCartney/Lennon will be one example of that. Maybe Radiohead. It's usually for the future generations to decide.

I noticed, one typical trait of great composers is natural skill which often means unbelievable speed and quality at the same time. Bach produced a 20 minute cantata (i.e. a 20 minute opera) EVER WEEK for YEARS (while still doing lots of other stuff). Schubert produced lieder like a machine, many of them the best of the genre. One has to contend with the fact, that there are only a couple of people in a generation with that ability and it probably cannot be taught and is to a high degree inate. As can be seen in many documentaries now, the Beatles came up with some of the musically best songs of the 20th century in hours or few days.

So lets enjoy the great works in the forms of the past and the great works in the forms of the present and lets not try to force past forms on todays audiences.

[1] https://www.youtube.com/watch?v=Y47JdUI_PhE