From the demo it seems you eventually have to co op to take down the bigger stronger monsters. Is this true?
I haven't had a chance to fire up MHW, but no other MH games has forced you to play co-op. The hunts get A LOT more difficult as you move into the later ranks so you need to get better with each encounter to learn their mechanics, equip specific weapons/armor to take advantage of the monsters strength/weaknesses, bring proper gear into the hunt (flashbangs/sonic bombs/traps/etc.)
I think they would piss off a lot of people if they forced co-op to clear end game content. Most people in the MH forums play solo, and actually take pride in taking out big hunts with no help.
The only time I ever co-oped in MH games is when I'm after a specific loot drop and going after kill count to get the drop.
*Edit*
Also, playing co-op adds a level of uncertainty to the hunt. Monsters will target random people and throw off positioning so you end up in the wrong place at the wrong time with your attacks. In previous entries you had 3 lives per hunt, and when you co-op you share the lives. So if your team mates sucked they could end the hunt for you, even you haven't been hit yet. So if you get teamed up with people that are under equipped, you could be 1 shot away from tanking the hunt.
In past games, I even ended up turning off my Palico partners since they screw up Greatsword positioning. I would get into position for a Charge Attack and before I could land a hit, the Palico would pull agro and the Monster would head in a new direction and screw me up.