Mostly principle. The internet is designed for end-to-end connectivity; let's strive for a more decentralized internet by giving big cloud and residential users equal access by removing NAT.
As for actual advantage, I can think of reduced configuration burden since you don't have to maintain two sets of firewall configs for dual-stack hosts. It's a small advantage only.
On the other hand, I'll be honest with you, there are disadvantages. As recently as 2021, people are still discovering problems on IPv6-only networks that necessitate writing new RFCs to mandate new behavior. Yes I'm talking about https://www.rfc-editor.org/rfc/rfc9131.html It's because of the low prevalence of IPv6-only networks that changes as fundamental as Neighbor Discovery have to be proposed in this decade.
I think that's a nice framing for the issue! IPv6 adoption is really slow, considering that I've been hearing about the necessity for what seems like two decades now.
As for actual advantage, I can think of reduced configuration burden since you don't have to maintain two sets of firewall configs for dual-stack hosts. It's a small advantage only.
On the other hand, I'll be honest with you, there are disadvantages. As recently as 2021, people are still discovering problems on IPv6-only networks that necessitate writing new RFCs to mandate new behavior. Yes I'm talking about https://www.rfc-editor.org/rfc/rfc9131.html It's because of the low prevalence of IPv6-only networks that changes as fundamental as Neighbor Discovery have to be proposed in this decade.