In theory you can save quite a bit on AWS costs by having instances that can only use v6.

But in reality at the moment there will probably always be at least one thing that only works with v4 a lot of the time.

Incentives are misaligned as well - it saves you money as the EC2 instance user, but the owner of the website you're trying to access has to support v4 anyway so they don't have a big incentive to change anything