As bikes frames get smaller, they get fitted with smaller wheels in order to keep the geometry as close to optimal as possible. That is why when frames get very small, they go to 24" wheels. Otherwise, the head tube angle needs to change too much to keep the front wheel from hitting the down tube or causing toe strike. As it is, the head tube angle is reduced on smaller bikes in order to fit wheels. The seat tube angle also gets steeper on smaller bikes to fit the rear wheel and I think in order to create the proper weight balance front and rear. Check out the geometry charts and you will notice the change in angles as the frames get smaller. A smaller wheel also allows for the head tube to be longer, which is better. 26" wheels are fine. As others have mentioned, there are numerous tire options in 26" and they are easily sourced. 650b would not be my choice for a touring bike - no way. Too few options, quite expensive, and difficult to find. Get a bike that fits you well with the size of wheel that makes for the best bike. It sounds like for you that would be 26" wheels and that is just great.