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).