That's sound advice. It is fun to make bags (and anything else you can) it's true. Not all bagmakers are ex-messengers though, my wife is but she was a seamstress (in clothing) and designer beforehand. Her actual specialty is dressmaking and we hope to have a shop for her someday. We have had orders from firemen though, they are always on the lookout for gear. I really don't like it unless it's an individual item, the stuff they use called Nomex is high-tech but my issue with it is it costs $65+US a yard so it's hard to keep around for the occasional order.
It's all about application too. When a special order comes in for a specific type of bag meant exclusively for carrying something pricey the customer knows they'll have to pay a premium that'd put the price of a messenger bag to shame. It's not the materials at that point but the 8-20 hours of design/construction they're paying for. Our largest messenger bags only take about 2 1/2 hours to build if there is no flap art but that's a pretty long time compared to many. A Timbuk by comparison takes about 20-25 minutes. Their time is reduced by some excellent assembly line function but also by the basic design/lightweight construction involved.
There is certainly no secret as to what a bag is made of, that's why all bagmakers tell it loud what they're using. The secret is having a design that is tested and proven on the street. It takes years to test and all of that time you get feedback and find small adjustments here and there that make all the difference. Take any of the top bag companies products now and compare them to what they had 4-5 years ago and you'll see the differences. (except companies that have been around long enough to settle and stick with a format already good or bad. Good ones keep evolving, Baileyworks for instance has a new design after 15 years or so in the biz)
Some recommendations if you're going to make your own bag:
Nylon 69 is the heaviest thread a home machine can use and the lightest thread for use making bags.
Take it easy on your home machine! Don't push it too hard or you'll need a new one quick.
Anyplace you sew through both liner and outer body is comprimised so think about how you'll attach the two together.
Add something, subtract something, good or bad not making someone else's bag = extra style points.
It's harder than it looks/sounds, expect to take 20+ work hours on your first bag so don't quit unless you aren't having any fun anyway.
Consider attaching your mainstrap by hand, a home machine isn't really up to it without regrets.
Have fun with the colours. Match it to your ride?
Sounds like I'm giving away the goose? I don't mind, we can only make so many ourselves and why be uptight when you're in demand? Good Luck.