A criticism of reducing the number and size of bags that you use when touring is the lack of volume you have to carry unexpected loads like extra food or water. It's always good to leave some spare room in your bags and I leave half my Ortlieb free for extra food and cans of soda etc. But that's just 3 or 4L. Using large dry bags gets around the spare capacity issue to some extent. I have used a 20L eVent compression sack to carry my gear, but actually only used half of it's capacity, leaving 10L of extra carrying capacity if needed. Old fashioned Carradice type saddlebags also have the ability to expand by using the long flap to carry stuff on top of the regular load and there are also D-rings on the top where items can be strapped. Obviously there is a limit to the capacity, but they are options for carrying things like extra food and a couple of 2L soda bottles. But what if you need still more carrying capacity. The next thing to do might be to stuff things in your jersey, but you could also bring along one of those 2oz nylon back packs that compresses to nothing and would allow you to carry another 10L on your back. But even if you don't have one of those it's easy to tie up the bottom of a rain jacket tie the arms together and make something that can be worn like a messenger bag. It would make riding a bit uncomfortable, but for a few miles it would be ok and if you needed to go longer you could take relatively light items out of the saddlebag, put them in the messenger bag and load up the saddlebag with fluids.

Of course if you were actually planning to ride somewhere you knew that you'd need to carry multiple days of food and water you should make specific plans rather than going all McGyver.