A hammer working fine today, that you own, is better than a machine tool tomorrow, that you'll have to pay.

I wish we had an idiom for this. Maybe something bird-related.

Also, developers who don't know how to get their daemons to listen on multiple interfaces at once.

That's not necessary; listening on the wildcard IPv6 address without the "IPv6 only" flag enabled on the socket is enough. A IPv4 connection arriving at it is mapped to a special IPv6 address, so the code doesn't have to care about IPv4.

Unless you were using Windows, where IIRC the IPv4 and IPv6 stacks were separate, and a single socket couldn't be used to listen to both at the same time. (This might already have been fixed in more recent Windows releases.)