> Oh I want to run a ftp server on my backend? Open up port 8000 on my firewall and forward to port 21 on my FTP serverfunny you mention FTP where that is distinctly not true as the application level protocol encodes IP addresses and ports to either (depending on PASV mode) peer to open for the data transfer.
If your server is behind a NAT and the user is using passive mode, it'll tell the client to connect to some internal ip address, so unless the NAT router does deep packet inspection and alters your packet on the go, that won't fly.
Conversely, if you disable passive mode, a NATed client would have the same issue because in that case it would tell the server to connect to some internal IP on the client's side which too won't fly.
Same issue for all other protocols that have IP addresses in their payload. There are very few of them these days for precisely this reason, many early media streaming and VoIP protocols were doing this too.
Also somewhat related: Port forwarding from one public address only gives you the ability to forward to one specific server. What if you want to run two different HTTP servers on your backend? What if you want to run different SMTP servers on your backend.
Now you're again down to needing packet-contents inspection or you need multiple public IPs in the first place plus a more complicated NAT table. With v6, all you need is to open a few ports.
And these were the technical issues.
There's also a political issue: As v4 addresses get more and more scarce, so increases the control entities with addresses get to have over what services they do and to not allow on the network.
Do we want to live in a place where no new service gets to participate in the internet? Where the next Netflix can't launch because none of the providers want to have yet another service competing against their own content business?
In order for the internet to continue to grow, we need an abundance of addresses and the only way to get that is to have wide-spread v6 support. And in order to get there, every single bit counts: Every service that can offer v6 should. Every provider that can offer v6 should. Only this way we can avoid one big cause for a very much locked down internet in the hands of the providers and the old guard.
v6 plays a very important role for both technical and political reasons to the point where we really need to fight the "v4 works fine for me" attitude. Having a v4 address to run a service on is a privilege. Don't argue from a privileged position based on lazyness.