There is a Filson advertisement from the 1910s that reads, in roughly the same typeface they still use today: "We guarantee every article we manufacture to give satisfactory service, and money will be cheerfully refunded for any goods that do not." The advertisement is from before the Second World War. The guarantee is, in substance, still in effect.
This is the part of Filson's marketing that the brand does not need to oversell, because the entire history of the company has been a slow accumulation of evidence for it. The Original Briefcase has been in production, in this configuration, since the early 1980s. The pattern has been refined. The bag has not.
"A lifetime warranty is not a marketing claim. It is a manufacturing decision, and a very small number of brands still make it."
It is, importantly, not the most fashionable bag you can buy. It is not waxed canvas in the precise way that waxed canvas is going to be next year. It is not the bag a stylist would put on a runway. What it is, instead, is the bag a generation of working photographers, journalists, surveyors, and tradesmen have used for forty years and replaced exactly zero times.
The repair argument
Filson's restoration department will, for a fee that is meaningful but not absurd, re-stitch, re-leather, and re-line a bag that you have owned for decades. They will not throw it out and sell you a new one. The aftermarket on these bags (secondhand examples from the 1990s that have been beaten into something better than new) is one of the genuinely encouraging things in this category.
You are not buying a bag. You are buying a forty-year relationship with a manufacturer that has structured itself, against the entire shape of the modern market, to keep that relationship going.