So grandma is going to be running trackmania server? You can play the game without having to run a server on your box, and you can also play on other players servers again without having to open up any ports. You can even book free server time can you not? And rent servers? If your not bright enough to figure out how to forward a single port.
and where did I say anything about running servers ? maybe if you read what I posted and/or knew how trackmania works. Trackmania allows you to make and share tracks and in this case, more importantly skins for your car. in order for other people to see your car skins you need an open port for the trackmania P2P sharing system. otherwise your car only shows up as grey or with a default skin for others, or if you're running with a custom car model then you also need this.
as for Grandma, people of any age who aren't computer knowledgeable run TM , people who I would never let near the router config.
for this and similar stuff, UPnP is a great solution for them. And for people with large families who don't want to manually map ports for their 5+ computers in their house that keeps needing or changing the ports they need.
and as long as the router/software isn't affected by this bug exploit, there's no reason not to have UPnP on, after all if the malware is already on your side of the firewall, you've already lost. and UPnP on or off will have pretty much no effect.