Amazon really needs to put a ton of work into making v6 work for everyone on the server side or this is a very big price increase on the low end.
If they had a compelling case to do the devops work and then everythings fine, I wouldn't mind this at all. The reality is a ton of stuff is ipv4 only (cloudfront origins, albs require ipv4, etc etc).
They realistically need free NAT or free 6to4 as a transition plan.