For network protocol perspective, FTP is weird old protocol from current view. Forget Active mode since it's no luck under NAPT. Even in Passive mode, it requires NAT device to rewrite address in command, won't work with firewall closing unknown external port without dynamic filtering by packet inspection, and server configuration isn't simple depends on network configuration. Modern network environment like NAT64/DNS64 need special support for FTP. There are no reason to continue using ancient protocol forever.

How is NAT the browser's problem? You're about 10,000 words short of "excruciating", and about half a dozen reasons short of "a variety"

>There are no reason to continue using ancient protocol forever.

You undermine yourself with statements like this. I explicitly asked for excruciating detail. This response essentially sums up to "because I said so".

...and it's empirically incorrect, too: You can find a ton of reasons where people need FTP today if you look through this discussion. So many that I'm not going to bother repeating them here.

Even less of a problem now once IPv6 gets everywhere finally.