Lets assume a few things you get 18 75+ people together and they are able to kill La Thaine Worms in 10-20 seconds per party/group(DDs, Mages) so that would be 3 monsters every 10-20 seconds (sounds like an efficient group). You've already capped lights on most people so almost all monsters are dropping chests. Lets also assume the person running around popping the chests is making sure not to waste keys so they are checking them before popping. Now lets account for lag on their part and say it takes them 5-10 seconds to check a chest.

In 1 minute at 20 second x3 kills that's a total of 9 chests.
At 5 (assume 6 w/ Key Macro) seconds per chest that means a person popping chests can open 10 chests per minute at peek performance. This leaves them 6 seconds to do something.
At 10 (assume 11 w/ Key Macro) seconds per chest that means a person popping chests can open 5 per minute leaving 4 unopened.

You can argue you don't need to check Red chests but to that I say people come and go for various reasons.
You can argue you don't need to check Gold chests if you're after EXP/Cru/TE but to that I say the moment 1 person takes a KI/item and others notice drama will ensue.
You could argue well everyone can help pop chests but to that I say it tends to lead to multiple people checking chests which leads to kills slowing down for various reasons which means EXP/TE/Cru slow down as well.

I guess if the level 75+ is a mage they could toss out a spell in those 6 seconds they have to spare since that doesn't require running up to a monster.

So how is a level 75+ key person any less of a leech than the level 30+ person?

Neither of them really have time to do anything else assuming your group is decently efficient.