> There is no such price
If it’s sustainable at $1/month, and a customer is willing to prepay $1200 for a 100 year subscription, that sounds sustainable to me.
But that's a gamble! That's exactly my point. Someone is betting on something. How long the customer will stay, how long the software will be maintained and available, etc.
It’s sustainable at $1/month at current costs, but those costs will go up over time. The subscription price could be raised accordingly but they can’t go back and ask for an inflation adjustment from people who bought it.