Consider that the conversation started before we had real numbers, and that estimates generally were somewhere between 2-4 million copies.
Add in that nobody that I can recall suggested the GT would get axed straight away, but that if GT7 (or I suppose GTS now) were to sell similar numbers then that might put Polyphony in the position of getting shut down.
I don't think that logic is particularly flawed. If GT6 had sold 3 million, and then GTS sold 3 million, that would probably be fair cause for Sony to have a serious look at what was going on.
We know now that GT6 sold more than that, and that GTS is likely to be somewhat of a departure from the traditional GT formula. Which could well be a reaction to the reception of GT6, or it could be something they've been planning for some time. We'll never know.
Even if GTS doesn't sell particularly well, Polyphony will likely simply be told to go back to making simplistic RPG racers with huge content lists, because that's safe. At this point, I don't see that there's any real danger to them until after GT7 at least. They would have to bomb GTS and GT7, which is always possible but seems unlikely.
On the other hand, production costs will be going up as they expand their team, and real support for an online focused game won't be cheap. They're not like pCARS, where the budget was only $5 million. You don't have to sell much to make back $5 million. Polyphony probably pays more than that in yearly salaries, not counting all the other costs of producing such a game, and so the bar for them to count as successful is much higher.
One of the issues with being a big seller is that the costs of producing such a game are high, and so magnifies any result. Successes turn into big successes, but failures are correspondingly large too. The most pCARS could lose was $5 million (and not even that, considering their funding model), whereas any given GT game stands to lose $50 million or more if it goes tits up.
I think one of the other contributing factors was the assumption that Polyphony would refuse to adapt. Fueled largely by the evidence of GT6 largely failing to adapt to GT5's largest design flaws. From what little we know, GTS at least seems to be attempting to break new ground, so I think that for now at least the idea that Polyphony would keep remaking GT1 until it killed them can be discarded.
The whole conversation changed once we got real sales numbers, but users are still quoting back to posts before that. It's not helping. Although the topic question still stands up well, and I think it's fair to say that it's definitely possible that GTS was affected by the relatively poor sales of GT6.