I agree, bunch of small bags gets old quickly if you frequently take a train for example. There's also a very good chance you accidentally leave something behind when you have umpteen little parcels to move from bike to train and back. But I still like to carry my gear relatively evenly distributed around the bike, and with low center of gravity. Panniers are a good way to achieve that, especially if you have a lot of stuff. A friend of mine rode some 40 kms with all his gear in a large hiking backpack. Think about it, just one bag, easy to carry off the bike, great ergonomics, fits in trains and buses, nice compartments etc. Sounds like a good idea. He says he's not going to do it again, ever.

Trailer is one option. You could theoretically stuff everything in one big drysack and tie it on a pair of wheels (or, just one wheel). I doubt it would make planes, trains and buses any easier, plus it would be a major PITA trying to find anything in there. And there's the weight question: right now your system weighs a bit under 4kg empty. Flatbed trailers are 6+kgs and upwards, plus the drysack.

As far as space goes, you can add to 4 Ortliebs by getting a handlebar back and/or the Ortlieb drysack that rides on top of rear panniers. This is apparently not a solution for you, if you dislike 4 bags already you'd probably hate having 6 of them. But it's doable.

--J