Having services be accessible on a link-local address and then advertising that service via mDNS is a completely legitimate use-case that works extremely well and is extremely common with Apple devices amongst others. The advantage being that it still works just the same even without a router handing out addresses or if you just connect two devices directly to each other.

Also what gives you the impression that zones were “deemed a mistake”? They may be awkward in URIs but they are very much not a mistake, they are a deliberate part of ensuring that each link has its own link-local subnet without any ambiguity. It solves the problem of what the operating system should do if you need to access a link-local address that shows up via more than one network interface, which is a very real problem with unscoped IPv4 link-local addresses.

Finally, ULAs don’t and were never intended to replace link-local addresses, they serve a different purpose entirely.