I have no doubt that one day it will happen (someone will chase 450/500). Of course it will at some point, but it will be an exceptional event, requiring a stroke of genius (like Astle's 200 a few years back) as opposed to something that might just happen if they have enough overs and apply themselves. For example if someone gets going, in a test the captain can just chuck everyone on the boundary and wait for them to mistime something (like England did to Astle).
And in this last test, even with 2 full days England were never a chance. They were never ever going to realistically have a go at it. Even if they tried to do it at 2.5 an over they would have had to have survived 180 overs (three new balls) on a pitch that at times showed swing, turn and variable bounce. Never going to happen.
Oh and as to your last point, Australia have been scoring at 4-5 an over for years, they "revolutionised" test cricket ages ago (10-15 years-ish), particularly with Gilchrist, Hayden etc. Yet, they have still only chased over 400 once (in 1948 with Bradman).
Would have to wonder how many opportunities they have had to chase 400+ however...
edit: beat me to the punch