IPv6 isn't just a larger IPv4. There are features inherent to it, like link-local actually functioning and being predictable, unlike APIPA in v4 which was grafted on as an afterthought and breaks more than it works.

It also functions router-less. You can grab 30 10-port switches and just stick them together and start plugging computers in. It will work without configuration or an authority.

I am all v6 internally, but that's not because I have a splatillion devices, but rather it's just better and easier to manage.

16M devices on one network would almost certainly have major scalability problems all its own. SMB chattiness alone . . . shudder.

Having the breathing room is great.

You have two teams that independently set up private networks but now someone has to talk to them both?

In IPv4, they likely stepped on the same private subnets. In ipv6, they pretty much certainly did not step in the same ULA prefixes. My VPN setup is a mess of a maze to deal with the fact that most things I connect to are all independently allocated 10. subnets, with the IPv6 focused customer being easiest.

Also, if you want to embed information in your addressing, like vlan I'd or room information.

Besides, you can have addresses like fd37:5f1a:b4c1::feed:face, and that's fun isn't it?