Well, it is amazon himself that brags their premiere was 8 millions, and only talk about this and only bring this all the time, if they are not talking about the rest is clearly they had a huge drop off from episod 1-2.
They even said people came to watch the "final battle" but most didn't came back after it
But like people mention, its only 15th in the top 15th if you count only originals, and unlike netflix, like people mentioned, people had subscriptions for free if they paid another service.The fact it made it onto the top 15 which is usually dominated by Netflix means it can definitely be considered a success in terms of being a streamed show and Amazon seem happy with their metric of it bringing people to their shop.
You could say it since it enter that chart, it was a corporative success i guess, but a failure in everything else.