+1 on traveling light. The author did mention this, too: "
Your enjoyment of a trip will be inversely related to the weight of your luggage. Counterintuitively, the longer your trip, the less stuff you should haul. Travelers still happy on a 6-week trip will only have carry-on luggage."
It's certainly not always possible if you need real dress clothing or real hiking/camping gear. (I've never gotten to the point where I can do a month-long trip that includes a long distance week-long+ walk along with some fairly formal evening wear in a carry-on.)
But you can probably get closer than you think. By myself, if I'm mostly just traveling in cities with "business casual" as dressy as it gets, I can travel almost indefinitely with a 40L travel backpack.
For men, mixed business/pleasure trips to countries with a conservative dress code can be tough (Japan for example). I have found that higher quality suits have better fabrics that are more able to recover from being stuffed in a small space. The problem I have yet to solve is shoes. In the West, not only are suits for meetings becoming rarer, but dress sneakers are acceptable almost everywhere, and they can do double duty for leisure. In Japan I would feel underdressed if I wasnt wearing standard black leather shoes. Maybe it will change with the new generation. Things are certainly lighter than in the early 2000s and 2010s, when there was a distinct echo of the infamous Burleson dress code[0]. (Though the cultural disdain for tattoos still exists.)
The last couple of trips I took to Japan were for events with essentially a developer/marketing oriented organization and I was able to comfortably get off with pretty much business casual. Maybe I wouldn't have felt as comfortable for a customer event.
The last trip I really felt I overpacked for with respect to train/transit was some time in London, followed by a long-distance walk where I really needed everything I had with me, followed by an ocean liner return where I needed jackets and real dress shoes--which, as you suggest, even Rockports aren't really general purpose. You always want more shoes than you can reasonably carry.
Wheeled vs. non-wheeled luggage is definitely a tradeoff. I lean towards non-wheeled in general and just maybe take a few more taxis if the luggage is too heavy to schlep around the city a bit.
