Light years ahead on the drivetrain is a bit of a stretch. Hence why the LBZ/Alli 1000 is still considered a viable and one of the better/best drivetrains to date.
Without getting into details, because if you know the changes, you know the details, from the LB7/Alli combo to the LML, they were the same basic combination and internals. Longest run of a largely unchanged combo of the big 3.
L5P and 10 speed is the biggest overall jump or departure from the now 20 year old combination.
Chassis, yes. From the GMT 800 to current 4th Gen there is basically nothing carried over.
And what do you think a new 4th gen loaded HD 6.6 Duramax Suburban would cost? Makes the older Duraburb not seem so bad (in general, the queer steer model that started the discussion is not as desire able).