I.e. address exhaustion pressure isn't only about the complete inability to buy an IP it's about how much work you have to do to use IPv4 addresses.
AWS charging for IPv4 will help with the former, because companies now have more of a reason to care about IPv6. But it might delay the latter, because fewer IPs wasted on internal servers means less price pressure on useful IPv4 use by ISPs, giving those that still hold out less financial incentive to change anything.
That percentage should be well above 99% for a popular website operator to even consider going IPv6-only; needless to say, in most markets, this figure is way lower, with no clear path to improving.
The only way I can think of getting the behaviour you describe is if you configure the network to hand out v6 addresses and routes to hosts, and then just black hole the packets. What you have then is a broken network.
If your network has broken v6 then yeah, that's gonna cause problems. That's not v6's fault, that's down to having a broken network.
If everything host on AWS becomes dual stack and gets first class IPv6 support after this move, that's a win for IPv6.
The price for v4 address is going to bob up and down as more networks and content providers adopt v6 and are able to reduce their public v4 usage to just their edges.
There will definitely be a long tail of v4, especially in the corporate world, but I think we'll get to 90-95% v6 capable within the next 5-10 year