I don't post or even read Fictional all that often, but I draw the line between ideas that are realistic vs. ideas that are dumb on their face.
Discussing alternate numbers for I-99, or giving interstate designations to the toll roads in Oklahoma or former toll roads in Kentucky, are things of merit and are actually feasible changes.
Proposing a freeway blasting through Yellowstone NP is one of several possible examples of nonsense proposals.
A serious idea deserves serious discussion. A truly ridiculous idea deserves ridicule. A little common sense goes a long way.
I just don't see the harm or why it's so wrong. I think it can be valid and enjoyable to discuss fictional situations so long as it's not literally impossible, like building a bridge from California to Hawaii or something. For the people who want to discuss building an Interstate through Yellowstone, why not let them? The goal isn't common sense or realism so it should not be a concern. It's like walking up to a hot dog stand and demanding filet mignon.
I for one have discussed with friends, drawn up a bunch of weird maps, drawn up roads, and imaginged what would had to be cut out for I-95 to be extended to Key West. Of course, that, in real life, is an absurdly stupid proposal, but there's no harm in imagining it for the sake of imagining it. I enjoy it. If you don't like the thread, then just don't participate. Let people enjoy things, y'all. Nobody owns the hobby/interest of roadgeeking and doesn't get to decide what it is for somebody. It's toxic. There are plenty of more ridiculous threads with less substance than FritzOwl on this forum that people never have a problem with, like in Off-Topic.
If the entire internet was ran by this forum's members, nobody would have any fun. A lot of creative content on the internet is exploring fictional and impossible situations. There's no realistic or convincing timeline in which all the nukes on Earth are set off or Hitler takes over the world or if everything was free, yet there's videos on that that are enjoyable. Plenty of conversations I've had with people I know revolve around talking about made-up situations and what they'd entail. I don't see what's so different about some of the threads in Fictional Highways.