THEY ARE OBJECTIVELY PLAUSIBLE AND OBJECTIVELY A SERIOUS CHALLENGE.
But what are your criteria for objective plausibility ? What's the yardstick ?
Clearly, it can't be that you think that they're objectively plausible - that's just another iteration (a restatement, really) of subjectivity. What's the measure of something being objectively plausible ?
Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?