No idea why people say it should be free, it connects to a cell tower, it's like saying MIFI/tablets devices should be free. Yes its part of your data, but it still requires a number that is connected to your account. eSim still gets its own number to connect to the network, but that number is not used for anything, but your actual phone number is routed so all numbers on your numbers sync account get forwarded to the main line. So monthly payment is totally fine and justifiable.
No idea why people are complaining about that it needs postpaid. It is part of a wifi VoLTE SOC feature. So it makes sense carriers do that to get people to migrate to post paid.
HOWEVER, the 10$ monthly charge is ludicrous. iPad add a line is 10$ that is totally reasonable in fact it seems cheap now if the watch is also 10$. All wearables used to be 5$ and now that apple watch came out they hiked their prices to 10$. Yes, I get it more people will be using it, but still, only data you will be using is streaming Apple Music (whoever has that) and talk/message when REALLY needed. This is not a phone replacement, you would only use certain features when either forgot the phone at home, or going for a run/hike/workout. Even then you would not stream all that much. because battery would run out real quick.
I personally think carriers will realize and either, hike the prices on their tablets/phones add a line, or reduce the price to 5$.