Agreed with the above. But. There will always be the potential to be squeezed on time for the briefing because some unforeseen thing came up to be dealt with (or a bunch of small ones) before TOD.
And there will will always be the potential to have a lapse in monitoring, and/or some cascading mode confusion, where when you finally "reconnect" with the airplane, the speed (or some other parameter) is not at a good spot. We can (and should) be as diligent as possible with the time management, and with monitoring, and with mode awareness, but none of this makes for a surefire catch-all trap for all situations that may be encountered.

There must still be the outermost layer to trap the bad situations that leak through: the comfort and ability to fly the airplane, that allows someone to calmly lower the nose a few degrees (to the 2000 feet lower altitude they were cleared to) without panicking and shoving the yoke and having a PIO that injures flight attendants. And there is only one way to ensure this comfort and ability as a baseline state.

Millions of flights happen each year. It is inevitable that a runway change will happen or a new STAR, etc. It is not necessarily an error to end up making a briefing after top of descent. I don't know what happened in this incident but I doubt they just waited until a fairly low altitude to start the briefing. It is more likely that the first mistake was not monitoring their instruments, which should be done by the briefer as PM.

Both pilots should be monitoring the 'plane.

Yes, of course there will always be runway changes, requiring a condensed re-briefing below TOD, but the implication that both or all pilots will be head down, engrossed in the plates and not regularly glancing at their PFD is what I find so difficult to understand about this incident - especially during a significant change such as a level off.

You put the plate down for a moment and watch the automatics capture the new FL and the thrust levers come up, then continue.

That should be well covered in your CRM course from your company...