A traditional touring bike needs to handle well with and without luggage, act as a normal, everyday commuting bike and a weekend fun bike and fit into normal bike-shaped spaces on trains etc.
These days, when people have more than one bike, tourers are becoming heavier-duty and more specialized. I can see a role for longer chainstays and long racks to accommodate more luggage. Simply adding a longer rack shifts too much weight behind the rear axle.

Unless you use a trailer, you need to distribute a heavy load around a bike so multiple bags are unavoidable.
When flying, I wrap them all in a plastic sheet and ductape it into one package. I try to keep all my luggage on the bike and just roll it along for as long as I can inside airports.
One of the great advantages of travelling by ferry from the UK to mainland Europe was the hassle-free handling of all my gear. It just stayed on the bike.

If you are carrying a load that exceeds 4 panniers then I would advise a trailer. They have disadvantages and can be tricky to manhandle over obstacles eg steps.