I don't think Yamaha screwed up with their introduction to the big ADV market. I just think they were aiming for a pretty niche market in the first place.
They wanted to build a bike that was going to be a GS killer. They spent a lot of R&D money to design a purpose specific engine and bike that, if judged by quality and reliability alone, could stand toe to toe with its Bavarian brethren. At the same time, they had to hit a price point that would make the bike appear to be a bargain to someone who was already predisposed to buy a BMW. And there is where I believe the trouble started.
People who are looking at BMWs obviously already have a fair amount of disposable income at their fingertips. They're mentally adjusted to the idea of their dream bike costing $20,000 and up. Those same potential BMW buyers are certainly aware of the status of owning a BMW; if you already have the disposable income for a really expensive bike, then the perceived status of the bike you're buying becomes just as important of a selling factor as the quality of the bike (actually, it might be more important, at least for some). Why pay a wheelbarrow full of cash for a bike unless your bike is going to project the image that you crave?
And along comes the Super Tenere. At face value, it seems like it should be a winner. Japanese reliability at a lower price than the market leader. But who exactly are they trying to appeal to? That very narrow range of buyer who has the money for a BMW, but is willing to spend 5 grand less to get a brand new bike with no status, no cachet, and certainly no bragging rights of ownership. If you're looking at a BMW, then it's a safe bet that all those perceived qualities of the BMW (including status) are important to you, and you're willing to pay for them. If you're that BMW guy, why would you pay less? And if stone axe reliability without frills was what you were seeking, you probably wouldn't be looking at BMWs in the first place, even if you had the disposable income. I'm that guy, as I imagine a lot of guys on this forum are, but being on this forum just reinforces our confirmation bias. We have to acknowledge that sales of the GS trounce sales of the S10, and that's not because Yamaha screwed up their marketing. Yes, they could have built a cheaper bike, and positioned it to compete with something like the DL1000, but how would a bargain basement ADV bike draw away potential BMW buyers who were already willing to spend twice as much on a bike? You may think that the S10 was overpriced upon introduction, but to make that claim, you'd have to have the inside track on what the development costs were on a totally new bike with an engine that wasn't going to be shared across other platforms.
I think that Yamaha faced the same hurdle at making a competitor for the GS as Japanese bike makers in general faced when they spent decades trying to dethrone Harley in the cruiser market. They made a far superior product to the Harley in areas that you'd think would be important, yet in the mid 2000's HD was selling a quarter of a million bikes a year. It didn't matter how much of a bargain a Japanese cruiser was, or how much better it was, or how much they advertised them, because HD had long ago mastered Lizard Brain Marketing 101, and the Japanese couldn't compete with that. BMW holds much that same position in the ADV market.
I don't think Yamaha screwed up the marketing for the Super Ten. I don't think anything they tried marketing-wise would have succeeded in doing what they wanted to do, which was to substantially make inroads into the GS buying market. They could have tried to make a cheaper ADV bike to try and compete with Suzuki, but in 2010, how big was the "adventure bike" market anyway? Why spend the money to develop a bike to compete in a low cost, low interest segment of the market (at the time)? How many DL650s and DL1000s were being sold in 2010 that would have made Yamaha say "aha, now that's the market we need to get into"? I think that proposing that S10's would have flown off the shelves if they had only priced them cheaper ignores some of the market forces at play, both back when the S10 was introduced and now. Yamaha specifically wanted a piece of the GS market, and they weren't going to get that by making a bike that buyers would perceive as "cheap' or "bargain level", even if it was an excellent bike. A DL650 is an excellent ADV bike, but no one is going to make a claim that it's a serious competitor in the demographic that buys BMW.