Re. your first point: When working with NPS, you would not actually assume that 1.000 people who voted with the highest possible value actually translate to 1.000 actual recommendations. I think not even the most vocal proponents of NPS would claim this. What matters is the _trend over time_, and that trend allows drawing conclusions regardless of actual individual customer behaviour, unless you have reason to believe that what customers actually do is completely independent of the score they give. If today 50% of respondents rate positively, and tomorrow 25% do, for large enough pools of respondents, something has gotten worse and it's not a stretch to assume that the total number of recommendations that go around for your product will go down.

Re. your second point: You are right that it is not explicitly asked for, but that does not imply that the core assumption would be faulty.

So much depends on the actual product being sold, and many other aspects, but being an imperfect metric or only a rough approximation would not imply that the metric itself is garbage, as the comment I replied to stated.

Here's a simplified example: Let's say I own a web shop, where growth implies a growth in sales. Someone on the positive end of the scale is more likely to shop again (contributing to keeping the current growth rate stable), and more likely to recommend my shop to others (contributing to positive growth). Someone in the middle or on the negative side is less likely to shop again (contributing to negative growth) or even actively recommend against my product (counteracting any positive growth the promoters would cause).

Does the NPS tell me anything about _actual additional sales_ I can expect? No. Does it tell me anything about _actual customers I will lose_? Also no. But it is one predictor of future growth, and as such useful.

Does my speedometer tell me if I'm driving in the right direction? No. Does it show how many traffic jams are ahead? Also no. But it is one predictor of when I will arrive at my destination, and as such useful.