Are we really comparing the role of Huey in Memphis to Mama Rose? That's absolutely ridiculous. Half the time when Huey is onstage, he's not even singing. He has some really high notes, but there is certainly room for a breath in there as well.
There have always been demanding roles in musical theatre. Look at Bobby in Company. Onstage nearly the entire time, difficult musical range- Raul burned himself out doing it, and that show is decades old. Fanny Brice in Funny Girl? Same thing. If anything, that's the closest comparison to Huey in that she sings some difficult songs but gets large breaks of dialogue. Lastly, how about Finch in How to Succeed? Another role that barely ever leaves the stage with some vocal challenges, also decades old.
The roles have always been out there, but so many actors nowadays go into shows unprepared, and I'm sure the belting and rock-musical tone of many of these scores doesn't help either. But to say these 'never leave the stage' roles are a new thing? Absolutely not.
"Are you sorry for civilization? I am sorry for it too." ~Coast of Utopia: Shipwreck