One of the problems of Surly as I see it, is that they are rather faddish.
I'd have used the word niche rather than faddish but I know what you mean.
What they have managed to do is sell niche product to a mass market, I'm sure somewhere they'll be part of a business studies syllabus.
Their making was probably the LHT, was anyone else in the US offering a stock steel, fairly traditional tourer when it launched in 2004? Apart for maybe Trek. I read someone in the US saying if you typed "Touring bike" into Google the first 5 hits would be Surly LHT'ers, maybe that's no longer the case and one of the reasons it was dropped.
Then they seem to have popularised the idea of off road touring, yes I know it's something people have always done, but it hasn't always been possible to buy an OTP bike designed for it from any mainstream dealer.
Then this whole fat bike thing, dismissed by many (Including me) as a fad, still going strong a decade later. I admit I still don't get it, but then...
Have they had any real failures? Bikes released that flopped and were withdrawn after short production? I can't think of any, though they may have passed me by.