People want to play new content. People want to do new dungeons when they come out, new raids when they come out, etc. Regardless of their skill level, people want to check out the shiny new thing.
On the flip-side, I do think having actual challenging solo content is good.
A solid middle ground is to simply have Normal and Heroic versions of Solo Challenges. Normal Mode can a bit challenging, whereas Heroic version can be soul-crushing.
I’m hopeful that the Jailer’s Gauntlet is reflecting that Blizzard is not giving up on solo content. I would love to see the next expansion add new solo challenges to pursue for players designed with all types of players in mind, not just either the most casual or most elite.