I get your point, but those tow numbers are notoriously optimistic. Most people I bet would not be comfortable towing 4000 lbs with a Maverick, and it would struggle on grades or in heat. You can even feel that kind of weight with a full-size truck. Above 5000 lbs in most places you need independent trailer brakes.
The real issue that limits the Maverick for a wider audience is the rear is too small to comfortably fit kids, especially in car seats. Adding 4 in of leg room to the rear and making the whole truck 4 in longer would've made in a great homeowner family option without sacrificing much agility.
Most people don't need to tow 4000 lbs period. If I had a Maverick and needed to tow 4000 lbs I would absolutely do it though. I've towed more than that in an older Tacoma that's not that different from the Maverick. Would I do it at 75mph? Probably not. Would I be towing 4000 lbs going 65 up an 8% grade in the heat at high altitude in Arizona? Again probably not, but the idea that a small truck needs to be able to do everything is just against the concept of a small truck. If you must have the ability to do that, get something bigger.
I agree that the Maverick's bed is small and the back seat is small. IMO they would have been better off making a regular cab or an "access cab" thing with two doors and fold-down seats, and used the extra length to add to the bed. Those are great if you're single or don't have kids, and you just need to carry passengers very occasionally. If you're regularly hauling kids around you definitely want the next step up. A lot of tradesmen essentially never even use the passenger seat though, and the back seat is just lost bed space unless you're using it for locked storage.
The real issue that limits the Maverick for a wider audience is the rear is too small to comfortably fit kids, especially in car seats. Adding 4 in of leg room to the rear and making the whole truck 4 in longer would've made in a great homeowner family option without sacrificing much agility.