Which brings me back to 'what was the goal for the series', and again, I still think it was to set Prime apart from the competition. Draw in a decent number of people who are hesitant to sub to prime, and keep some of them. You'll never retain 100% of new customers you bring in. A substantial bump in sub numbers, though, would mean a shitton of increased revenue. Spend money on one end, make more money on the other.
This is why new sub numbers are very important to gauge the success of a huge show like this. Which is why social media engagement is important, and how the show resonates with the public compared to other shows, especially comparable shows that run at the same time. Which is why 'number of views' compared to 'existing customers' is important, and which is why '100 million views' doesn't mean it's a success, but probably means that it wasn't. Because even if every single view of those 100 mil was on an individually subbed prime account - which it wasn't - that means that less than 50% of all people who have access to the show decided not to watch it. Mediocre attraction, which incidentally corresponds well with the mediocre reception the show got.