Composite wings can be unbelievably strong and design practices have gotten a lot better in the last 50 years. That being said there are a lot of challenges with composite aircraft, especially since related to first part qualification testing that validates all the tooling is correct and there aren't wrinkles and other defects
If this thing does anything more than test flights I'll buy you a beer. I agree with you; this thing will never launch a payload. The recovered F9's are now a proven concept and by the time this thing is ready to compete Elon will have his costs reductions in sight with a technology that has 100+ flights under its belt.
I'll bet it will do at least one proof of concept launches but zero commercial launches.
Mmmm....I dunno, man. The Spruce Goose never did any proof-of-concept troop hauling. It got up off the water, made some scary creaking noises and never left ground effect. Power-to-weight, the thing is a 747-800. Size-wise, the thing is a 747-800 and a half. But there, where most planes have a fuselage, Stratolaunch has a stress concentrator. And it's carbon. Which means nondestructive testing is super-duper tricky and any delamination and it's done. Rutan's a clever guy. But if you recall, the last big scary cargo beastie he built scraped its winglets off on takeoff and the closest he's ever gotten to a production aircraft is the Beechcraft Starship. ...and he retired in 2011 so this is his posse, not him.
All points more valid that my idle speculation. My thought is anyone who has enough money to build this but never use it has enough money to fire a rocket or two into orbit. And maybe the sci-fi geek in me is just secretly hoping they do.