Demoman is fine. If you can't kill him don't say that he's op. If you see a crit rocket coming it's better? Or a tanked backburner pyro that can kill a heavy from behind in less than a sec? Every class has it's strengths and weaknesses.
The only thing that's annoying facing a demoman is the nade spamming at a map like dustbowl. At cp maps there isn't any problem.
Valve disagrees. They call stickies winbombs. That is a clear indication i'd say. 1 crit sticky launched near the feet of a heavy (which isn't hard to do at all) will kill him. Or alternatively usually 2 normal stickies do the same job. Effectively forcing the heavy to stop firing if he wants to have a chance at living. Stickies are just far too usefull.
Every class can get killed, but Demoman has an advantage over just about every class but the spy. Everything else just place stickies and blow them up or retreat to safety if they are smart enough not to run into it, of course you would have to see them coming.