So I started this comment as a reply, but I felt like people would also like to hear about this as someone who cares about mud rooms when pretending I can afford to buy a house window shopping.
I grew up in a rural/small town area where mud rooms are still highly valued to this day. Small houses will also occasionally have mud rooms, even if it’s just a weird little hallway. In this area, the mud room usually has built in shelving and enough space to put a shoe rack plus sometimes the washer/dryer. Most commonly, it’s about the same sqft of a full bathroom if the washer and dryer aren’t in there; only a little bigger if they are.
I usually see them with hard wood, tile or linoleum floors to make them easy to clean, and a cheap, rough rug from Menards gets thrown on top to wipe and leave shoes.
Another, and probably more common, thing is a covered and insulated all-season porch. Usually it has screened, cheap windows and spans most of the length of the side it’s on. This has less insulating power than a mud room, but with the trade off being a socializing space while semi outdoors and bug free. In the 150 yo house I grew up in, it even had a bathroom because it was a farmhouse. This house didn’t have any screens, it was honestly more like a mud room than a porch; however there’s also a local terminology thing about size being important in the distinction between mud rooms and porches.
I mean I agree with you, but in my small Midwestern town, it’s cheaper and easier house maintenance to have a mud room instead of constantly cleaning the floor in front of the door. I think houses in the cities will see them disappear, but not in places where the towns are ~300 to 50k people. It might become hard to find in small houses in the Midwest, but it will still be there in most mid sized houses