I have seen many people today say that something isn't challenging if it can be made easier with gear. This is how WoW and pretty much any RPG have always worked. Even Dark Souls, the game that people praise as being the prime example of challenging gameplay, can be made easier with gear.
The new Solo Challenges are no different than any other form of difficult content in the game. At lower gear levels, it will be difficult to beat it. As you play the game and acquire more gear (whether that be in later tiers or later expansions), it will become easier to do.
PS: Most of the Solo Challenges can be done without having to commit to a raiding schedule to get gear (My warrior is currently Ilvl 892 just from Nethershard Tokens and Mythic+).