I've never really understood the need for Epic level adventures, I mean 20th level is already so crazy powerful. I mean the party literally starts their day plane shifting into any area they want, and basically can just curb stomp and geniuely high encounter.
I did a 20th level run where the party fought through Asmodeus' lair, and killed him. It was an incredibly epic adventure, I honestly couldn't imagine how to go any "bigger".
So in other words, 20th level is already so over the top, what more is really needed? I think the Epic Boons add enough little extra if you want to push it, but that's really way more than you ever need.