You mean "enable them all over again for every new DHCP assignment, unless you insist on static IP assignments".

Why wouldn't I use static dhcp?

Forwarded ports are not always static, we're not in the world of just web servers and SSH.

Different devices may need to use VoIP, P2P, games and other applications that cannot be strictly mapped to just one system or even just one port. UPnP handles dynamic mappings, so you don't have to update your port forwards every time.

What’s wrong with static IP assignments? Doesn’t this solve the issue?

Story time: It depends on the hardware at your disposal. I'm now on the new T-Mobile Home Internet service, the router+wifi device supplied (a Nokia 5G LTE based unit with a SIM on one side) firmware has basically no configuration - you cannot assign static DHCP, no bridge mode, no port forwarding - it has UPNP on or off, that's it. A truly sparse webUI, frustrating no-config device at 1.0 firmware level that doesn't even show you what the DHCP ranges in use are. My G-Shock watch has more configuration options than this thing does. :-/

You? You should. Random consumers on the other hand, are usually not capable enough to set up port forwarding, or manage a network.

UPnP might not be the right solution, but there is a clear and definite usability issue when it comes to networking and online gaming.