I think you're overestimating the "appreciability" of art and music. I would say most people don't have the training to understand or enjoy the majority of the most important critical works in art or music from the last 100 years, if not longer.
I think you're gonna need to unpack how you define "most important" and "critical" here.
It's quite easy to see most people enjoying a lot of art and music (I personally wouldn't include much 'understand' in my own definition of 'appreciate' here).
The more you study something the more you can have nuances or less-widespread angles by which to appreciate it, that will lead you to appreciate different things, but that's not the same as requiring that study to appreciate it.
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From another angle: critics have claimed that the Beatles were less important than various less-popular or more-niche musicians of their era since the 60s (from many different angles, even then). Is there a point where sustained higher popularity negates that criticism? Does time overwhelm niche more-highly-trained criticism at some point?